The recruiter handled himself professionally and should be commended for not being bullied. Mr. Zhang sounds like a punk, and would probably be better served by pulling the image.
Especially since this is a UX research internship, and the candidate made perfectly clear that that is how the recruiter was coming across, so the recruiter had every opportunity to clarify.
The candidate didn't handle it perfectly, the recruiter however handled it far worse.
Early phases of job applications are about following directions and saying what you need to truthfully say. I'm actually surprised that my original comment was voted into oblivion -- its a pretty obvious reality. The recruiter did his best to say "Just write Unix" without crossing a line.
Facebook markets itself as hiring the best people. Therefore, when applying to Facebook, you want to market yourself as being among the best. When a recruiter first tells you that Linux isn't Unix, you assume that the elite recruiter looking for elite people is looking for an elite response which explains the difference.
OP's point is that image of Facebook elitism is just smoke and mirrors.
This is a genuine question; I often can't think of how to phrase things and I think an example would be educational.
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 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.