Where did you get that from? Sources?

Google rolled their own VCS, because Google is older than Git, and they needed something that works. Their custom VCS is a hacked up version of Perforce.

By the time Git came around, Google was already pretty much committed to their in-house custom tool, too many things relied on it.

The main thing that has been developed in git to allow very large repos is shallow clones (both in terms of history and slices of the repo). This model works well enough within git's logic, but it's just historically not been focused on until fairly recently (and I don't really know what the state of play is there - I think there's still a limit at a certain scale where simply finding the state of play of a large checkout becomes a bottleneck, and you start to want a persistent daemon to use FS notifications to keep track of what's changed instead of stat()ing every file in the tree)

(I've often pondered if it would be possible to make a DVCS where there's no firm repo boundary at all, i.e. you could construct a checkout from any combination of trees and commits stored in different locations, and have it work seamlessly. There's probably more than a few thorny issues in there, but it would be an interesting concept)