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.
email 1: I want to apply
email 2: I fixed my resume
email 3: my fix didn't work? you must be wrong
email 4: you are still wrong
but at email 3 you clearly aren't getting it and should ASK them what they want
This is a genuine question; I often can't think of how to phrase things and I think an example would be educational.