The problem with this argument is that it doesn't really explain why large repos are wrong. It turns out the choice between monorepo and multi repo does not have a one-size-fits-all answer. Microsoft can make their own tooling, and internal tools don't have to run outside of Microsoft so they're easier to develop.

Google reportedly used to use Perforce for their monolithic codebase, and Facebook is supposed to use Mercurial with a bunch of modifications. They all have huge code bases mostly in one repo (I've heard Facebook had a >50GB Git repository, and Google's codebase is supposed to be in the TB range).