> Pretty sure GNU coreutils really does intentionally deviate from POSIX compliance in a handful of places, otherwise POSIXLY_CORRECT wouldn't exist.

To get UNIX certification, you can just patch it to make POSIXLY_CORRECT=1 the default.

Or even don’t patch the utilities, and just patch /etc/profile to set POSIXLY_CORRECT=1 globally.

UNIX certification requires that the system have a mode of operation available which passes the test suite; the existence of config settings which if changed from their defaults produce standards violations is not in itself a standards violation.

The point is that the default build of GNU coreutils in the default configuration is not POSIX compliant, not that it can't be made to be POSIX compliant. Obviously it can be done, otherwise that environment variable would not exist.