I used to work in a monorepo of that size.

All of the PRs I ever submitted touched a handful of files in my project’s subdirectory.

That's effectively an answer of "yes".

Or what "yes" looks like to you? It can do all the work itself, for a 50m-file monorepo, without a human guiding it which files to look at?

If it were true then human programmers would have been considered obsoleted today. There would be exactly zero human programmers who make any money in 2025.