The BSD's on the other hand could probably apply as they are one system (kernel and userland) with one governing body. But I doubt they really care to play the corporate standard mark game. They are UNIX and have always been so.
Not SCO, the Open Group. The Open Group is a not-for-profit industry consortium that maintains various standards and conformance programs for those standards, among which POSIX and UNIX are probably the most widely-known.
Novell gave the UNIX trademark and test suites to The Open Group; all SCO actually got was the right to sublicense the copyright to the AT&T UNIX software (but not the actual copyright itself). The former is much more important than the later because there are operating systems which pass the test suite (and hence are allowed to use the UNIX trademark) but which aren't based on AT&T-derived code (e.g. macOS, z/OS, EulerOS)
Note that Inspur K-UX you mentioned is no longer a certified UNIX because the vendor failed to pay the renewal fees. The same is true of Solaris.
Apparently, “conformance statements” are different from “registered products”. I guess one is self-reported and the other is externally verified?
Products get dropped from the register if the vendor fails to pay renewal fees. The product no longer appears on the register page, but the page on the product is not taken down and can still be directly accessed. This is what has happened to both Inspur K-UX and Oracle Solaris (and I'm pretty sure a few more over the years).
I would think the BSDs have at least as much Unix heritage as macOS, but they aren’t certified (“Certified Unix” is a rare breed. https://www.opengroup.org/openbrand/register/ lists only 13 products)
(Nitpick: it isn’t Unix™, it’s UNIX®)