My comment is in the context of 'their own Scheme implementation'. Since they already handle some .scm files internally it's not that wild to pick Scheme over other languages as the base for plugins. It's a power user editor, Scheme should not scare them.
It can't really be simpler than Lua, come'on. Maybe if you already know Lisp, but for people who know C-like languages (Java, Javascript, C++) Lua should be much more "obvious".
https://docs.helix-editor.com/configuration.html