I regard "traditional Unix" as the 1970s era. There were no sublicensees. You bought the source code from AT&T. That was the only option.

You then likely hacked the source code to make it work on your machine.

I can understand that others might include the commercialization of the 1980s and the Unix wars as part of "traditional Unix", but even then, my IRIX system ca. 1990 supported end-user kernel extensions, to handle new types of devices. Our imaging hardware was not "connected with well-defined text-based interfaces".