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.

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