As a serious emacs user, this looks like a stripped down version of emacs. There's no reason to use this if you're already using emacs. Its UI even looks like emacs, with the modeline and minibuffer.
I had similar thoughts when seeing that it uses Web Assembly to provide plugins. It they rewrite the core functionality with that and add an editor then they've basically reinvented emacs (though with less flexibility because the plugin code is compiled). As an emacs user I find the plugin sandboxing idea interesting because security is basically missing from emacs packages. You just have to trust or review what you're installing.
Terminal support in emacs isn't super great so there's definitely room for improvement there. Maybe there's even room for Zillij and emacs to integrate via plugins. That could be quite interesting. I still use emacs and tmux side by side without any integration.
