That doesn't sound true to me at all, except maybe in some very small niches. I've used Bitbucket at exactly one job; I've found Codeberg, but no project I've used was actually hosted there; and literally everything else I see or use is on Github.
A bunch of larger projects have a mirror on GitHub for easier access.
BTW there's launchpad.net which is often overlooked, bit it's vital for Ubuntu-specific projects.
At paid day jobs, I had to use BitBucket at least twice, and I miss better code review tools, like Phabricator.
GitHub definitely dominates the market, partly due to the network effects, but I don't think they have a lot of moat. If something goes badly enough wrong there, there will be plenty of viable alternatives with an easy to trivial migration path.
Their moat is a billion development tool vendors that have "integrate with Github" as a must-have and expected functionality.
It's overlooked because in true Canonical fashion they went hard in on their not-invented-here-syndrome VCS that nobody asked for or wanted. That and also the integration with Ubuntu and nothing else.
There is also a lot of stuff on Gitee (China), but due to langauge barrier, it's hard to judge.