By default it isn't UNIX certified (as you mentioned, it doesn't quite make sense since it is considerably different). However z/OS with UNIX System Services, is what was certified I believe.It's interesting seeing z/OS here since it looks nothing like what we think of as "Unix,"
https://www.ibm.com/docs/en/zos/2.3.0?topic=zos-unix-system-services
There is a POSIX layer for z/OS too: https://www.ibm.com/docs/en/zos/2.4.0?topic=ulero-posix
It does make me wonder if they paid enough, could something like Cygwin on Windows get certification?