There is no citation but he seems to be pretty good source :)

https://news.ycombinator.com/item?id=13560051

It was stated in Saeed Noursalehi's talk "Scaling Git at Microsoft", held at Git-Merge 2017. Until the conference recordings are available, here is the closest thing to a "source": https://twitter.com/no_more_ducks/status/827479795185364993