Yeah for sure there's little in common and I'm sure it was just corporate legal hostility. But I'm sure the general experience of developing a 16-bit machine was educational. And I'm sure the choice to go with the 68000 (or NS32k which they also evaluated at first at Atari) was likely informed by the annoyance of having fiddled with the Z8000's awkward segmented architecture.

Re: POSIX, we Atari users were already crosscompiling things to MiNT (that MultiTOS was based on) back then before MultiTOS came out. In 1990/1991 I ran a setup (off floppies!) that had a bash shell, uucp, nn, etc all set up. Friends of mine with larger hard drives and TTs were using GCC and so on. This was all just before Linux came out and before I moved to a 486, which I only did because Linux was an option there.

Eric R Smith was the author of MiNT and was hired by Atari in their latter days to work on TOS and make his work "official." Sort of a last gasp effort after neglecting TOS for years, and the last few OS releases for the Falcon and TT were actually pretty compelling and competitive in some ways with what was on the Mac, etc..

Later, MiNT was all open sourced under GPL as FreeMiNT and it's still very actively developed today. You can set up a whole open source Unix-like environment today on FreeMiNT + EmuTOS complete with bash, login, initd, etc. RPM package management, whole thing.