For a small number of people, it would look like "It's your oncall week, and you unexpectedly receive a large diff stack from a complete stranger". Most engineers at Meta won't touch anything related to C++ during their tenure there, and you can safely assume all this stuff would be abstracted away from them.

If you are on a C++ project, odds are that your experience here would be "I'm on-call and got pinged by a random person saying they were submitting a diff stack to implement this feature". Except if your project was sufficiently large, you'd probably have known about this ages ago so the diff wouldn't actually be unexpected, nor the submitter a complete stranger.