I've been seeing people, never ones who were actually there, make this claim in the last five to ten years. Rc was on tenth edition Unix. I've seen no evidence that it originated on Unix. The manual calls it the Plan 9 shell.
Pehaps this is misleading.
The mk paper (and Unix man page) doesn't mention rc.
Plan 9 existed since the mid-80s.
This sentence in the Unix rc(1) sure makes it seem like rc was Plan 9-first:
Environment entries for variables are kludgy for UNIX compatibility.
So rc was made for Plan 9. The examples in that paper are definitely on tenth edition Unix, I think this would be because
A fairly complete version of Plan 9 was built in 1987 and 1988, but development was abandoned. In May of 1989 work was begun on a completely new system
from https://doc.cat-v.org/plan_9/1st_edition/designing_plan_9 (1990).