Sure, but that just means UNIX vendors had access to the source code.

It doesn't mean end users had access to the source code.

In fact, vendors would have been prohibited from distributing AT&T source code to customers who weren't themselves AT&T licensees, just as CSRG was prohibited from distributing BSD to customers who weren't licensees before 4.4BSD-Lite.

The same goes for most other proprietary source-available products. For example, game developers can't open source Unreal Engine projects even though Unreal Engine source licenses are available to end users for free.