UNIX APIs are not implementations.

You compile programs for Unix using language implementations.

C++ implementations don't make any sort of special or unique accommodations for UNIX/POSIX APIs. For the most part calls into external libraries are firewalled and not optimized in a way that typically engenders undefined behavior, so POSIX APIs tend to work. But any external library would also work since it's not like POSIX is exclusively shielded or singled out in any particular way.

If a compiler for Linux time travels whenever you use SCM_RIGHTS, take it back for a refund - it's faulty.

You can try to request a refund for free software but the terms of the compiler explicitly state that undefined behavior is permitted to "time travel" and that there's nothing faulty about it. That's C++ for you ;P