At its core, Meta uses a modified version of mercurial. Most of the cool stuff I was referring to is not related to the core "hg" operations but related to the server-side hosting (monanoke) and the platform for operations like code review (diffusion), CI/CD (sandcastle), and all kinds of other tooling (notable mention: interactive smartlog for its fantastic GUI). I see no functional reason why a company like GitHub could not make the "Hub" part of GitHub better. It seems like they basically just haven't because they don't want to.