We can talk about the advantages of monorepos, but your questions is phrased in a way that makes me think that you don't see any "trouble" in multiple repositories.

I would encourage you to do some research and keep an open mind.

I see "trouble" in all approaches: their solutions to manage their git monorepo approach it by switching to multi-repo emulation.

They seem to have purposely left that out of the scope of this article. There are myriad articles about the benefits and downsides of monorepos vs multiple repos.

In another sibling comment I mentioned how I looked through their engineering blog and didn't see any post where they have talked about any of the benefits they are enjoying due to their use of monorepo.

The question is specifically about their usecase, since not everybody would hit the same bottlenecks as they did with git monorepos.

Iow, have they stopped and thought whether it's still worth it (eg how often do their engineers make use of the monorepo benefits like cross-project refactorings)?