Citation needed.

I'm not sure what you mean by "Unix specification". But if you mean the international standard POSIX, yes, people care. Red Hat routinely participates in POSIX spec revision.

There are a very few deviations where you have to enable "POSIXLY_CORRECT". If that's what you mean, then you can turn that on. But in every area that matters, Linux distros implement the POSIX spec by default, and you can even turn on the POSIXLY_CORRECT mode to exactly follow it. They extend beyond it, but that is allowed and expected.

The people who build the tools in Linux distros care a lot. I know the implementors of dash and GNU make routinely refer to POSIX. The Linux distros don't have to as much with POSIX because that is generally a conpleted work and it's the maintainers of the tools who must address the updates to POSIX.

The UNIX specification is not the same as POSIX: https://en.wikipedia.org/wiki/Single_UNIX_Specification.

You might say "their exact view of what UNIX is isn't important and POSIX is," but POSIX is not the UNIX spec. You might think the Unix spec isn't important - and it really isn't today. Linux generally targets what is important and what users care about - and that isn't the UNIX spec. It is often the same as the UNIX spec, but not always and there are deviations.

Posix is a subset of the Unix standard - it's necessary, but not sufficient to pass Unix certification.