How does monolithic repos solve that. Surely people who fix bugs in a library aren't testing the entirety of Facebook every time (how long would that even take? Assuming they've even set such a thing up.)
I used to work at Facebook. They have servers that automatically run a lot of their test cases on every commit.
It is at least easier to correlate the changes. When you have X+ modules, you have potentially X+ histories you have to look at to know when a change was seen with another change.