I would strike “but did not originate from a Unix-derived operating system”. That may be a technically nice point, since it’s well defined, but its usefulness is limited by the extreme rarity of OSs that meet it. NetBSD, FreeBSD … is OpenBSD still around? MacOS is questionable, if the Berkeley part of it is only an appendage, to a Mach “microkernel.” There are plenty of others, but not in wide popular use. I’m sure Solaris and AIX are still around, but in this context, who cares?
“Mostly” POSIX.1 compliant raises as many questions as it answers. Isn’t it like being somewhat pregnant?