This was the same design goal that Windows NT had. In fact, it launched with Win32 (Windows), OS/2, and POSIX (UNIX).
I think the OS/2 subsystem was 16-bit OS/2 1.x so nobody cared and the POSIX subsystem was just compliant enough to win government contracts.
This design is why we have the "Windows Subsystem for Linux" (a name everybody hates) because "Windows Subsystems" were already a thing in Windows.
Docker, Distrobox, and even Flatpak are one kernel with multiple "personalities" but they are all still Linux I guess.
You can also argue have this on our desktops today with things like KVM in Linux and Hyper-V in Windows.
Back then if POSIX support was actually serious I would never have bothered to play with Linux, by buying the first edition of Linux Unleashed book.
I also bet that many others would not have cared, and used UNIX/NT personality if that was the case.