It's a broad generalization that isn't meant to be precisely accurate in all cases. I'm not claiming it's a great term, but it does succinctly describe a notable attitude and culture. If there's a better term to use that conveys the same message I'm sure many folks would be happy to adopt it.

For many younger speakers, "you guys" is legitimately a second person plural pronoun (like "y'all") and implies nothing about the gender^1 of the referents, even if they consider singular "guy" to be a synonym for man.

Some older speakers use "guy" as a term of address, as in "Hey, guy", similar to how one might say "hey, bud" or even "hey you".

I don't think it will ever happen, but it's funny to imagine something similar happening and "bro(s)" coming to be a nongendered term.

Anyway, it's never crossed my mind before that "tech bros" singles out men; for me it evokes a stereotype of, yes, men, but it's really an attitude, value system, world view, or collection of behaviors that are being alluded to. (Of course, it's also only implication in the sense of "hinting at", because it's not contradictory to say "tech bros are the worst, and tech women are too").

[1] The... non-grammatical gender. English no longer has grammatical gender in any case, so it's unambiguous, but it feels weird to use "gender" in a linguistic context and not mean grammatical gender.