It’s interesting that they prefer to develop such a tool rather than giving up on the monorepo concept.
One perspective on this is that many ORM libraries don't take the DB as the source of truth (one very good ORM library that does, and which saved me in this case, was JOOQ). I think a lot of small-scale problems could be solved this way, monorepo is just another variation of this solution: moving the source of truth into the repo.
It is surprising to me how often variations of this problem come up. Obviously, there are solutions from multiple directions: having cross-language definitions (ProtoBuf), Arrow (zero-copy abstractions suitable for high performance), maybe even Swagger which comes at the problem from documentation...but I think this problem still comes up anywhere (and the DB approach is, imo, a very strong approach with a decent ORM at smaller scale).
I think the core issue with git is that the submodule thing is obviously an afterthought that's cobbled together on top of the preexisting SCM instead of something that was taken into account from the start. It's better than nothing, but it's probably the one aspect of git that sometimes makes me long for SVN.
At the scale of something like Facebook you'd either have to pay a team to implement and support your split repo framework, or you'd have to pay a team to implement and support your monorepo framework. I don't have enough experience with such large codebases to claim expertise, but I would probably go for a monorepo as well based on my personal experience. Seems like the most straightforward, flexible and easier to scale approach.
My last place was about 10 years young, 150 engineers, and was still working within a single git repo without submodules.
There is a non-zero amount of discoverable config that goes into managing a repo like that, but it's trivial compared to the ongoing headaches of managing submodules, like you suggest.
The others weren’t “oh huh” enough to be easily recalled writing this comment, which probably speaks to their interestingness. But yes, you can chdir from search to calendar to borg and their dependencies, internal and vendored. It’s pretty much all there. It was pretty splendid, actually, and influences my thoughts on monos to this day.