Commenting as an outsider to both projects, one possible motivation to fork Helix might be this PR:
https://github.com/helix-editor/helix/pull/8675
Helix was originally meant to be a "batteries included" editor without any scripting/plugins. That PR intends to change this and so there is a bit of controversy around it. Overall it looks like the PR will eventually be merged.
The Mitos fork could have be made for totally unrelated reasons. This is really just my speculation.
helix has a plugin system in the works, but unsure when it’ll actually land. lots of folks are already daily driving it, myself included.
Treesitter grammar for each language does not count as plugin? I think I had seen some dynamic libs when I tried to build it…
From the commit history it rather looks like an ai agent speed run of a single guy frustrated about this not moving fast enough.