>The Git community strongly suggests and prefers developers have more and smaller repositories. A Git-clone operation requires copying all content to one's local machine, a procedure incompatible with a large repository.

They aren't copying all the files like you do with Git. They have a custom set up that sounds like it lets you checkout just the parts you need. I don't have time to read the whole thing, but it sounds like it works by breaking down a "super repo" into small "sub repos". This actually makes sense.

There is no way working with a 300gb git repo is fun or efficient, and they've probably been doing that for years at Microsoft.