Note that the project obviously isn’t ready for public consumption (it doesn’t build outside Facebook’s infrastructure, for example).
A few more details on Mercurial at Facebook here: https://code.facebook.com/posts/218678814984400/scaling-mercurial-at-facebook/ (from 2014)
I guess if Facebook is building it, they need, but I always figured DVCS was pretty good at fan out. They’ve made changes to speed up client access, but I would still expect it’s possible to funnel tons of commits to a single python server.