But even then, as you mention, Git doesn't scale well for them. This is why Facebook is using Mercurial. This is why the Chromium and Android projects are spread out over hundreds of Git repos. This is why Google's internal monorepo is using a Perforce-like homegrown tool backed by Google's distributed wizardry.
Git will start to see significant scaling problems beyond say 1 GB total data, 100,000 files, and X number of refs (there has been significant upstream work around scaling refs - a bunch from Twitter I believe - and I haven't measured how well newer versions handle thousands of refs).
The good news is that unless you are a software project with hundreds of developers, have large binary assets, or are doing something like Twitter and have all your developers push all their in-progress feature branches to the monorepo (madness if you ask me), a Git monorepo will scale for you. A Mercurial monorepo will scale even better. Keep in mind that most people aren't {Twitter, Facebook, Google, Mozilla}, so chances are you won't ever reach these scaling limits.