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.
It’s an extension from Mercurial as the blog post states.