It's a signal. It's not a strong signal, and you certainly should not base your entire perf on it, but if the number is unusually high or low, it's a signal that could warrant further investigation.

(I once worked with an engineer that had two PRs, both fairly small bug fixes, in a given calendar year, and when I looked more carefully, they did not have any other obvious output or impact.)

Trying to parse your sentence, which is ambiguous...

You're saying that the manager-of-managers would argue that the number of PRs should affect perf ratings? Or the MoM would push back against the line managers who were giving ratings based on # of PRs?