It is not Unix or even Unix-like but they added a POSIX compatibility layer so that you can use a variety of familiar programs.
When I think of non-Unix and non-MVS i'm thinking more on the line of the IBM i, PalmOS, or the Newton OS. All three are quite alien under the hood to anyone who grew up on a Windows/Unix world.
The main _inspiration_ was the Amiga, but not really AmigaOS.
BeOS was built in the still-new C++ but AIUI predates a lot of standardisation of C++ which subsequently happened -- as was Psion's EPOC32 and its later rebranding as Symbian.