Is fossil the main competitor to git? The "only" significantly different one?
this is a flawed comparison.
The major competitor to git is mercurial, and while git has won the popularity contest mercurial is still in use across large projects.
Java/openjdk moved from mercurial to git in the 2021 time frame.
I don't believe fossil has ever been used on a scale as large as openjdk from an organizational point of view (thousands of participants in the ecosystem, across dozens of jdk implementations, across more than a dozen hardware platforms, etc)
Fossil as used in sqlite vs mercurial/git openjdk repos as used across the java ecosystem - I think java is one of the projects where platform support is at least as wide as sqlite, if not greater, and the codebase is much larger and more sprawling.
Facebook has moved on from mercurial as well, and it's not to fossil.
Fossil is a great tool, but seeing it as "one or the other" vs git isn't useful.
In particular, this guide on the fossil website is a guide for users wanting to use fossil, so git is not the only competitor, but for the purposes of fossil adopters, that web page is a great resource!
Where did you hear Facebook is dumping Mercurial. Last I heard they were using Rust to optimize Mercurial.