A role might specifically require extensive AIX experience in particular -- and if it did you would say you needed "AIX experience" -- never some vague umbrella family of "UNIX experience."
It is entirely unbelievable that the requirements for this role would exclude your experience. Doubly so as they're recruiting from interns. Triply so if you know anything about Facebook engineering teams. "Production Engineer?" "Frontend Engineer?" At Facebook? Yeah, those teams run Linux.
ps: The folks telling you not to language lawyer are correct. But your initial assumption that the recruiter had made a dumb mistake was also correct.
Nope. The hiring manager meant that.
The recruiter in this case is a pattern-matching engine with no internal semantics, as a result of Facebook hiring them and training them but not educating them.
Eventually they'll notice that they are only seeing car salesmen with extensive experience in selling brown cars with walnut dashboards, rather than carpenters. [ https://news.ycombinator.com/item?id=15190438 ]
Getting a little frustrated with resume-screening-by-grep is not at all unreasonable. After the recruit had you play whack-a-mole for three resume revisions, a little bit of snark is to be expected. Don't let people walk over you.
It would have been pretty simple to tailor your resume in that regard. What I know of PE/SRE type positions at companies like FB and Google is that you are literally working with some Linux flavor nearly all the time unless you are applying for a very niche role.
I can claim experience with developing userland applications portable across POSIX-compliant systems, as well as some understanding of lower-level operating system concepts. However, to be honest it seems I am unqualified for the role as specified, which makes the exchange all the more embarrassing. Oh well, what's done is done.