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.
alt Hacker News
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.