"compatibility with unix" didn't matter in those days.
There were many, many OS's, all running on computers so expensive (for whichever micro/mini/mainframe segment) that most people didn't expect to replace them, ever.
cpm
vms
tops10
tops20
unix
multics
rsx11
mvs
os/360
gcos
apple dos
etc etc etc.
"compatibility with unix" didn't matter in those days.
There were many, many OS's, all running on computers so expensive (for whichever micro/mini/mainframe segment) that most people didn't expect to replace them, ever.
cpm vms tops10 tops20 unix multics rsx11 mvs os/360 gcos apple dos etc etc etc.