Neovim can run in server mode, where other editors send it user input and then Neovim sends back the buffer. This is how I use vim in VSCode — not the Vim extension but the Neovim extension, which uses the real Neovim, which of course reads my Neovim config and plugins and makes them available to VSCode. So it seems like helix “just” needs a server mode, and then you can integrate it into any editor.