It’s an extension from Mercurial as the blog post states.
Yup! In fact Meta’s implementation is open sourced as Sapling SCM and includes both Mercurial and Git support[1]. Internally the main repository use Mercurial although I believe a few legacy projects still use git.
From the documentation, it seems like there is 0 mercurial support. It's its own separate version control system that diverged years ago. You can either use the sapling scm client or git.
Oh I didn’t realize. Internally we use `hg` commands for everything so I guess it’s just the internal version that supports mercurial