Afaik, fb employs less than 20 people to take care of the monorepo.

It's actually the other way around. The cost of dealing with all the downsides multiple version control systems is way higher than investing in tooling for the Mono-Repo. This is mentioned in articles by Facebook and Google.

Source? Sounds like you're pulling that number out of thin air.

Isn't it reasonable to assume that FB/Google will do a cost analysis of mono/poly repo approaches and pick the one that is the most cost effective? At that scale they have absolutely no room for dogma; it's all about costs.