It actually is Unix, depending on your definition. If you define it as meeting the standard, it is. If you define it as being the same changed code base, it's not.
There is one group which owns an artificial, government-created construct: the trademark to "UNIX". That is far from the final word on what constitutes a Unix. The social meaning of words goes well beyond whoever owns a government-granted monopoly on them.
OpenGroup manages that social construct, the government stuff is just there to protect against misidentification and encroachment. The standards for Unix, Sockets, and LDAP were all transferred to OpenGroup to manage and they've done so.
A single entity generally doesn't get to manage the meaning of a word. The real meaning is in our collective heads, and Linux is a Unix for most people in the field. Even the ones (like myself) who are fully aware of OpenGroup and the UNIX trademark.
In fact I consider Linux a Unix specifically to annoy the sorts of people who think OpenGroup has the last word on what constitutes a Unix. ACAB.
