You risk creating an atmosphere which is toxic to your own game if you concern yourself with this kind of nuance of speech.
You have established that the action would have been completely permissible and legal with subtly different phrasing; the player was open, honest, and clear with you about the specific one that he was attacking, rather than being vague and saying "I attack the third one". He made his rationale known to you, which is valuable information for you to have as GM.
If you're the type of GM to warn people when they're doing something stupid, this kind of information is important to give you a chance to point out the flaw in their choice: "Boo the hamster has by far the highest initiative, but likely won't do a lot of damage because he's literally a hamster; Minsc the ranger, on the other hand, has a crossbow aimed right at your head, hasn't attacked, and probably has a held action. Are you sure you want to step on his hamster?"
If they'd instead explicitly said "I step on the hamster", you'd have had no real excuse to give that extra context, nor to double-check their thinking.
When speaking in-character, players already deliberately context-switch to filter their speech to avoid referring to game mechanics.
You risk adding a second, metagaming layer on top of this, where they have to speak in the character of "a game-player who is ignorant of the game they are playing", whenever speaking OOC about their game choices.
By requiring your players not to use "metagaming" terms, you are requiring them to focus on a metagaming concern every time they speak; "Is what I am about to say, referring to game mechanics? How can I filter it to not do so? I selected this mob based on their initiative, but should I go back and find some other defining characteristic to describe it, instead?"
"I attack the one with the lowest HP" is also acceptable. Sure, it's more common to say "the most wounded one", which is not couched in metagaming game-mechanics language, but either is fine.
"I attack the obviously smartest one" and "I attack the one with the highest INT" are also both acceptable phrasings, where the player might reasonably guess that information (when fighting a wizard and a bunch of mooks, for example).
The very concept of initiative itself is metagaming. It's a metagaming hack that lets everyone get a go at hitting other people; the alternative is to have everyone slug it out simultaneously and then resolve everything at the end.
The only reason to feel that there is a special case for this specific piece of information -- the initiative -- is if they shouldn't know it, and that the UI shows it to them is a bad thing, which they should be pretending it doesn't do.
If that is the case, you need to make this clear to the players. Because right now, it's reasonable for them to take this as something that they and their characters can reasonably infer from the combat: some characters are clearly more capable or better positioned to attack than others.
I'd be inclined to go the other way, though; to amend your own expectations that initiative might be something the characters would be unaware of, and instead treat it yourself as something that's obvious information for them.