Some clues:
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.
The first paragraph of https://archive.org/details/rc-shell/
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).
Pehaps this is misleading.