The idea is that because you could simply use open source mercurial to achieve whatever you needed, a company couldn’t build services around it and make money off of it, so there were no dollars around to promote it either and pay sales people to sell it to enterprises or sponsor GitHub conferences and hackathons, etc.
Large orgs are often biased towards spending money for things as well rather than being inclined to use free stuff like individual developers tend to.
Anecdotally I've experienced a number of times some exec has proudly announced they've paid for some commercial product for everyone to use because their salespeople have sold it as a silver bullet for some common problem, and then dev teams need to diplomatically explain that there's industry standard open source tooling that's better and nobody wants to use the commercial thing.
That's specious reasoning at best, and for multiple reasons. The bulk of software development platforms standardized and uniformly built their services on FLOSS, and version control ends up being a tiny version of the whole offering. In fact, the bulk of their business model is based on providing version control services as a loss leader, as none charge for it. They charge for ticketing, CICD, package hosting, Docker image registries, etc.
Also, some of them offerer Mercurial as well. Somehow Mercurial still failed to gain traction.
