AIX, HP-UX, DGUX, NCR SVR4, Motorola, NEC, etc are all derivatives of Unix. AIX, HP-UX are SVR2 derivatives. That's why you still see COFF (well.. AIX's Extended COFF) format in AIX. HP-UX's ELF like format allowed the use of an almost SVR4 feature, shared objects (except they had them even before Solaris came out).
So... all depends what is meant by "true Unix". All of the above can use the name Unix... so by that definition, they are true Unix.
Shoot even the awful Unix System Services on the mainframe uses the name Unix (and it's REALLY a stretch IMHO).
Maddog once said "Unix is a Linux-like operating system." So maybe Unix is sorta "false" and Linux is "true"