A lot of issues disappeared when better abstractions were found on top of POSIX.
That's a weird and derogatory way to talk about people who appreciate having a standard which makes it easier to write cross-platform software.
EDIT: To expand on this comment and make it more productive. You're right. If the goal is to have first-class support for both Windows and POSIX-like platforms, the right approach is to have an understanding of what your software needs to do, what the POSIX APIs make easy, and what the Windows APIs make easy, and build abstractions which make sense based on that. But man, do I appreciate that if first-class Windows support isn't top priority, I can support essentially every other widely used platform in the world (including macOS) by just writing against the POSIX API, and I can even get okay-ish Windows support by using a POSIX compatibility layer on Windows. So my point isn't, "People who want to have first-class Windows support but do so through POSIX compatibility shims are doing it right".
It was first known as DEC OSF/1, then Digital UNIX, then Tru64.
Previous to OSF/1, DEC had sold Ultrix, first on VAX, then on MIPS.
https://en.wikipedia.org/wiki/Tru64_UNIX
The wiki says that OSF/1 was first released for MIPS, but that was before my time, and it wasn't supported for long.
I did use Ultrix on MIPS DECStations in college.
When I say "POSIX fanbois" I mean "fanboys" (people that are unreasonably attached to this standard and try to use it everywhere without being level-headed about its usefulness). That probably doesn't include the people who actually wrote the standard, which are all (in my experience) pretty reasonable and realistic about the limitations of things they've created.
This is certainly a better way to put it!
But it's possible that they indeed mostly had the POSIX standard in mind when trying to create their software, possibly because that's what they were developing on (e.g. one of the *nix distros or something compatible) and that's what was available easily.
Support for Windows might have been a bit of an afterthought, or something that was added later. It's still better than nothing, even if not always viable.
If anything, it's even more problematic, because Windows is "wierd" and people go out of their way to create special approaches to handle it. For macOS, too many developers think it's "just posix, ya know, like Linux" and then walk into horrible compatibility edge cases.
AFAIK, only the versions of Windows that included the POSIX subsystem were certified, the last one being NT 4.0. Long time ago.