Onivim isn't trying to be a vim GUI, it is it's own product that is something like VS Code on top of libvim. They opted to go with the VS Code extension ecosyst

Onivim isn't trying to be a vim GUI, it is it's own product that is something like VS Code on top of libvim. They opted to go with the VS Code extension ecosystem which necessarily excluded vim extensions. Not affiliated with the project, or even really a user, but bought a license a while ago to support what I still think is a nice idea.