As a tutorial for those of us like me who sometimes stumble in situations like this, could someone who found the OP's responses to the recruiter unprofessional please post example phrasing of what the OP should have said? Let's assume the constraint that the OP will refuse to lie (so they won't just change their resume to 'Unix' instead of 'Unix-like').
This is a genuine question; I often can't think of how to phrase things and I think an example would be educational.
> Let's assume the constraint that the OP will refuse to lie.
This constraint is invalid. People need to use contextual awareness. The recruiter is most likely not a technical person, so splitting hairs won't help. And based on a bit of knowledge about Facebook, it's highly unlikely that a company that prides itself on building its own data centers from scratch will actually use a creaky AIX 7.1 for their backend, so you just "lie". Heck, it's not even a lie, it's a white lie which you'll tell the engineer interviewer down the line, while explaining that you have POSIX/whatever experience.
In my opinion this whole exchange shows a need for improvement regarding people skills for OP, at least in writing. Considering it's an internship, this was probably a good moment for a wake-up call.
This is a genuine question; I often can't think of how to phrase things and I think an example would be educational.