TL;DR: it's bad if you are using it to cheat, what exactly is considered cheating is up to DM's discretion, although fudging dice rolls is explicitly called out in the Cheating section (p. 6 from DDALDMG).
It isn't explicitly stated anywhere in the rules that it is allowed or forbidden. My answer is actually that it depends on the intention, as your question notes.
I will state my case. First, the DM is allowed to change the encounter itself. Wall of text ahead about changing the encounter. It's stated the section before the quote you made.
While the adventure provides suggestions on how to adjust an encounter to provide an appropriate challenge for your players, they are just that—suggestions. You may, at your discretion, make other adjustments to the encounter by adding or removing monsters. While the monsters you add may be different from those listed in the encounter or the sidebar, they should be
thematically similar. For example, if your players are encountering a group of zombies, adding a bunch of pixies doesn’t make much sense. However, adding a zombie or a ghoul might. Keep in mind that while the characters earn XP for these new monsters, the maximum amount of XP they earn for the adventure does not change. Remember to give them a challenge, but don’t make the adventure unbeatable.
Challenge Your Players. Gauge the experience level of your players (not the characters), try to feel out (or ask) what they like in a game, and attempt to deliver the experience they’re after. Everyone should have the opportunity to shine.
Keep the Adventure Moving. When the game starts to get bogged down, feel free to provide hints and clues to your players so they can attempt to solve puzzles, engage in combat, and roleplay interactions without getting too frustrated over a lack of information. This gives players “little victories” for figuring out good choices from clues. Watch for stalling—play loses momentum when this happens. At the same time, make sure that the players don’t finish too early; provide them with a full play experience.
Note that the DM gets permission to do a lot of things, if these are for the goal of making a better adventure. Fudging rolls, while not the best technique for adjusting things, is one. For example, you might want to fudge rolls to make up for the fact that you put too many monsters and created a too hard challenge - or you might want to fudge a critical when the encounter ended up too easy and you want to deliver a little more damage - these could be the replacement for removing or putting a monster, which you are allowed to do. If you are doing it to make the adventure better, it should be fine.
If you are doing it to "cheat" (or allowing cheating), you are not fine. The DDAL DM Guide has a section about cheating - usually about players cheating, but we should extend that to the DM. AL is based on honor system, so nobody is going to be constantly checking on you or whatever, but note that fudging dice rolls is explicitly called out there.
If you notice something amiss—either with the paperwork or during the game (fudging dice rolls, not checking off used items, and so on)—discuss it with the player and attempt to resolve irregularities. [...] You can ask a player to reroll a dice roll that isn’t obvious to the table.
So, as you yourself mentioned in the question, sometimes it's fine, sometimes it's not. Did the PC die in the encounter fair and square, in a balanced encounter, possibly due to his own bad decisions? Then yeah, I would call fudging his rolls cheating. Did the PC die because you mistakenly put 2 monsters more than you should have and the encounter got way too hard? Yeah, let's pretend that 2 was a 20.