This is a solved problem, and even if it isn’t, there’s entire communities dedicated to it. There’s literally no reason why Facebook needs to be in the business of reinventing the wheel from scratch beyond some dude trying to show “impact”. It’s a distraction from core business problems.
Hosting a gigantic monorepo for 25K concurrent users is so far from a solved problem.
It is a fork of Mercurial and there wasn't a community dedicated to making it scale to the scale Meta was reaching hence why they invented resources in making Mercurial scale.
There are very fundamental differences between a mono-repo and a bunch of repos for each "service" or whatever. Lots of tradeoffs. I've worked both and I can see the reasons for huge monorepos. They make a lot of things that were previously hard much simpler... The tradeoff is your tooling needs to be able to scale with growth of the company. And for a company the size of FB, dedicating an entire team to improving the tooling for their monorepo is well worth it.
Saying that Meta "reinvented CVS" means you either know very little about source control or you are just being purposely misleading. Neither is a very good look.