I don't recall it having one of those? Maybe you're thinking of running a Unix LPAR?

Either way, I looked it up in our internal documentation. It's called SCRIPT/VS, and it's part of IBM DCF. It's apparently not a direct descendant of RUNOFF, but instead based on RUNOFF back in 67.

Yes, but troff/groff is not the only descendant of RUNOFF, and apparently they're using a different one.