I don't know what you're referring to with that distinction. AFAICT they weren't testing for POSIX compliance but rather where the POSIX layer was being used in practice, which statistically was by system-level frameworks rather than the applications themselves on the OSes they tested.

The notable change between now and 25 years ago is that the POSIX layer tends to be used at the lower levels of system frameworks, while the programs that run on those OSes tend to be written on top of higher level frameworks and do not directly use the POSIX APIs.

This is fine and makes sense given how much has changed in the last 25 years, but given that the POSIX layer is no longer being used (statistically speaking) for what it was originally designed for, it raises questions about to what degree that design is still valid, and whether or not it should be revisited to better suit what it's actually being used for.

I don't think anyone has any answers, but it's always good to question assumptions and never let anything in tech become a sacred cow. Maybe POSIX is fine for its current role, but that doesn't mean people shouldn't think about it.

Disclaimer: I skimmed the article pretty aggressively, so I may have misunderstood something.