I’m unfamiliar with illumos so I went to their webpage and the very first thing it says is: > illumos is a Unix operating system Is illumos an actual Unix (like macOS) or a Unix-like OS (like GNU/Linux)?
Legally, NetBSD isn't actually Unix. The brand doesn't mean what people seem to think it means.
- Derived from Bell Labs unix source
- Legally allowed to use the UNIX trademark (AKA certified Unix)
- A unix-shaped OS (similar but not 100% the same as POSIX complacence)
and those things are basically independent. Most GNU/Linux are unix-likes but not derived from original unix code or certified, but there's been 1-2 that did get certified. The BSDs are (now quite distantly ) derived from unix source but not certified (although ex. UnixWare is IIRC). Solaris was all 3 but OpenSolaris and now illumos are obviously unix-like and still based on the original code but not certified UNIX™.
(Take all this with a grain of salt; I'm typing this all from memory and IANAL)