Yeah, this is something to avoid in general. There are some people who hear "girls" the same way you do, so "guys and girls" translates as "men and women." But there are also some people who hear "girls" to mean, well, girls, so this translates as "men and girls" and sounds very weird. As people get older (say, from 20-year-olds to 30-year-olds), the balance shifts more and more toward the latter and more and more of the women you talk to will think there is something wrong with you if you call them girls, even if you also call men guys.

So yeah -- seeing "girls" in a job ad would make me guess the workplace consisted mostly or solely of young men who weren't used to being around women, and I would only apply if that was what I was looking for. Whether you think that's a problem is up to you. (I happen to think there are good reasons that some women don't like being called girls, even in the "guys and girls" context, but it's sort of beside the point in terms of writing a job ad, right?)

Linters are normally used as style checkers, rather than as tools that enforce rules. So it's more useful to think of it is a "Hey, did you mean to do this? Would it be better another way?" check rather than as a sexism detector.

For instance, a sentence like "We're looking for guys and girls to join our development team" would get flagged by joblint. And that might give you pause, and realise that "We're looking for people to join our development team" is a perfectly good alternative to use instead.

Yes. Girls are female children. Presumably you aren't looking to hire children. Guys doesn't have the same connotation. It is informal, but not demeaning.

The transgenders would burn you down.

If you wanted to keep the same informality, "guys and gals" would be the appropriate way to do this. As stated above, girl is a female child.