Along with DEC Ultrix, and (kind of OSF/1, Tru64, etc).

There were lots of other proprietary BSD based systems in the 80s as well.

Ultrix forked off BSD 4.2. It was the Unix I learned first, and working with it when the civilized world was working off BSD 4.3 variants made life really awkward.

Ultrix was my 2nd unix, after SunOS 4.1 on a massively over-committed Solbourne. I preferred Ultrix just because the machines were in a closed lab, and "faster" because there weren't 300 undergrads trying to compile their projects all at the same time on an overheating Solbourne...

Yeah, certainly Ultrix was one of the closest. Many diverged heavily though - HP/UX etc.

I guess they're confining the history to non-commercial (albeit semi-closed source) versions.