Well, there's good news and bad news. The bad news is that it isn't really intended as a limitation that your player has to tell you anything about where they learned it. The good news is they also can't demand you tell them exactly what they want to hear. (Because Spout Lore's a basic move I'll just be referring to the relevant player as "a loremaster".)
What To Ask Your Loremaster
The purpose of asking "How do you know this?" is to help you, as the GM, be a fan of your players. You know, part of your overall agenda? You can flavor the information you pass to them in keeping with the background they pass to you... and, honestly, you've pretty much got to? The information you pass to them had to come from somebody or somewhere in particular, after all.
Now, sometimes it'll be pretty obvious where it came from - Fletcher's been hunting all around the immediate environs so he's seen the local flora, fauna, and foot traffic first hand, or Fightgar Stonehammer grew up hearing tales about the history of the Stonehammer clan and their allies and rivals. Sometimes you'll have doubts how a character came to learn something at all, but even in those cases, "how do you know this?" isn't an essay question your player needs to come up with the secret right answer to in order to make the move. There needs to be an answer that everyone at the table can believe in, and whether it comes from your loremaster or another player or even you the GM doesn't really matter, but it has to exist somewhere.
Maybe a more important question to ask is "what are you planning?" Because that 10+ result means you have to tell them something useful to their current situation. Spout Lore isn't a move for idle speculation - if there's nothing useful and actionable your players are hoping to learn, you can't very well give it to them.
But let's suppose in both those cases there is still something useful and actionable your players can learn. Maybe they've recently overwhelmed a pirate crew and the only ship still floating is really huge. Maybe Wizzrobe read the Visions Through Time, learned that the lich-lord Ossian VI was planning to snuff out the sun, and is trying to find some clue to a countermeasure. Cool. Knowing that is going to inform:
What To Tell Your Loremaster
So, if your Loremaster's schtick is that they're "the books guy", that gives you an easy out on a 7-9 or a 6-. Anybody can write stuff down in a book, that doesn't somehow make them right. You're likely playing in a fantasy milieu where half the creatures running around are supposed to be alive somewhere in the real world, according to various books ancient scholars wrote because they were trying to write a history of the entire world and in some places they didn't have anything but travelers' tales to go on.
But hey, even on a 10+ you don't need to give them exactly what they asked for. You need to tell them something interesting and useful relevant to their situation. So if somebody wants to pilot a huge ship all by themselves, it would be useful to know that:
Yeah, a ship this size needs a crew of about 30. There's just too much to keep track of, even for expert sailors - Terwilliger's latest Tales of Tragedy recounted one sad tale of twenty of the best and brightest of a naval academy who took one of these out on a rescue mission, because it was all there was and they were all there was. The ship was lost at sea with no survivors. Fortunately, between captives and surviving pirates you can get up to 40, and the knowledge that they all need to work together to survive should be something of a unifier. Should be. So how are you going to motivate people to repair the lines and get this ship underway?
Or if they want to know the words to speak to snuff out the sun in order to stop Ossian VI's ritual, it would be useful to know that:
Words alone aren't going to be enough. You recall a passage in Ethelvin's Mortalum Limitae, on the power of words to shape mana, and oddly enough he picked the sun as an example. Ultimately he concluded that words alone could not suffice but would be used to channel the power required, which might be obtained by sacrificing a king beloved by the whole world, a flawless ruby the size of your head, and the moon. It doesn't seem likely that Ossian VI has any of those, except maybe for the ruby, but if you can track down the font of power he has managed to tap, disrupting it would blunt the destructive energies of the ritual. Surely you know how to locate something as potent as that, Wizzrobe?
(It's important, when you finish talking to the players as a GM, to make a GM move and give them something they can react to right now. Offer an opportunity/put someone in a spot/turn their move back on them/play to a class's abilities, that kind of thing.)
But all that said, sometimes you want there to be mysteries, things the players can't just remember, things maybe nobody knows. So let's talk about:
X The Unknown
It's likely you're going to hit this when you're prepping an adventure or campaign front. Something that's meant to be kept hidden from the players because finding it out is the entire point. That's fine; just know that you have to deal with this somehow. If the answers are supposed to be in reach somewhere, just make a note for when you have to tell them the requirements or consequences and ask. Like:
I could tell you what this room does, sure; the cog-drakes build specialized structures out of quite simple parts, and this is a shaper module, which takes as inputs a schematic pattern and a supply of refined metal in order to create machine parts and pass them higher up. But in order to actually understand what the whole grinder tower is doing you'll have to find the maintenance room they always build into these things and look at the master schematic. Judging by their tendencies to counterflow with processing it should be lower down somewhere - Fletcher, you could probably track a maintenance crawler, which would get you there eventually, or does someone have a better idea?
or
You've got your speculations that the boar faction's in the right, here, but the only evidence both sides are going to accept is in the original genealogy, which was lost when the elves abandoned their original homes in what they now call the Mist-Eden. Are you going to volunteer to make the trip, or let one of the players in this succession crisis make the first move?
Or, if it's more complicated than just sticking a signpost in front of your players with TRUTH written on the arrow, you can write a custom move about what happens instead of providing purely actionable answers to a Spout Lore. For instance:
The Mist-Eden, the lost original homeland of the elves, has a history that was deliberately obfuscated by later generations, to confound and perhaps slay the curious. While you pursue the Mist-Eden and Spout Lore in service of your aims, the GM will respond as normal, but on a 10-11 will additionally hold 1-Ruse, on a 7-9 hold 2-Ruse, and on a 6- hold 3-Ruse and spend at least 1 of it immediately. The GM can spend Ruse in order to: [divers alarums and excursions here]
Or perhaps:
When you Spout Lore about the contents of the mysterious complex buried beneath the barrier mountains, treat any result of a 10+ as a 7-9 instead. No one has ever seen its like; the best the GM can do is suggest fruitful if risky courses of investigation or provide dubiously-helpful analogies.
You don't even have to hook your custom moves onto Spout Lore directly; if there are only a few notable sources of information about the secret your front revolves around, you might find it worthwhile to make custom moves about interacting with each of them.
