I imagine that your question is from one of two points. Either you're talking as a viewer and you're just wanting immediate connective association based on what you know, which that's not the same as being consistent with the in universe knowledge of the relevant characters. Or alternatively, I can understand the paranoia over insertion of partisan political issues especially with the problems of localizers re-authoring work to fit their worldview and stuff like that as the insistence of inserting gender neutral terminology can bring up worries of insertion of intersectionality ideology where it may not belong.
But I believe the reason they're trying to stick to a gender neutral thing is because of language differences and the adventurer being uncertain of the sex of the kid. Japanese is filled with vagueness or lack of confirmation of sex in their language, it's part of the issue with how a lot of misunderstandings can happen because their language is built in a way that opens itself up to have a lot of misunderstandings.
For example, even though Ore is a masculine way of saying Me, and Atashi/Watashi is a feminine way of saying Me. They're not locked to male or female use. A tomboy can use Ore for example and nobody blinks an eye at that. They just assume that they're more masculine. You end up having to assume a lot of things with Japanese. You might have seen this oddity with automatic tarnslations, like when you translate a comment you found in Japanese talking about how cute a girl is, but the translation says something along the line of "He's so cute". Well, that's because the statement had nothing in it to refer to the sex of the person and therefore the automated translation had to default to something.
The other way they could avoid repeatedly using They would be to just emulate that sort of vagueness in an alternative manner. Like the adventurer consistently referring to the kid as "This kid" or "That kid" and completely eliminate the need for the usage of he, she or they. Where even the loaded worries behind the last few year's pushes on controlled speech related to gendered terminology, doing so would by nature bypass that as that sort of way to address a person can still sound normal while also maintaining complete neutrality where now even the reader has to assume what the adventurer is assuming, which may actually be more in line with the possible intent of the writer, though that's a lot of guessing.
Also, as a reminder, the adventurer has specifically already behaved in a way that seems to indicate he's not certain whether the kid in front of him is a guy or a girl, but solely noted that it looks like a girl. Now, from an English speaker's viewpoint, I can imagine that you would make an assumption and move forward with that. The most normal behavior would be to then assume that okay it looks like a girl, sounds like a girl, talks like a girl, must be a girl and default to she. And while a lot of people don't want to face that reality, we always make assumptions and move from there even if we say we don't. Same for judging people, the reality is that we are constantly judging. But we adjust our assumptions over time or we do not deliver final judgement, which is the reality of what people actually do when they follow the general statement of not judging people.
But we have a slightly different circumstance here, where the trope of a beautiful man or beautiful boy who looks like a girl is very common in Japanese media. Playing off that uncertainty is very typical, so there's still the same odd chance that he just assumes it's a very beautiful boy and immediately starts thinking he's a boy. In which case, he would have naturally started referring to the kid as a he.
If we could have a clearer idea of what the end gag would be, we could then adjust the translation to have the adventurer assume that the kid is a guy if at a later time he's surprised that she's a girl.
But if in the future he's not surprised that she's a girl, then the translation should be that he assumed she was a girl and it just ended up being correct.
My assumption then is that the translators probably don't know what the end result of this point is yet, so it kinda just defaults to using They to try not to commit to one assumption or the other until we finally get to the reveal, if we ever get there, in which case then maybe the translations could be retroactively adjusted though unlikely to occur because that's a lot of corrections to be made.