Huh? It isn't because of inability to handle load, that they host on GH.

Gitea performs alright. It can serve surprisingly-heavy traffic from a potato-tier VM with a SQLite backend (assuming the host network can keep up with it, anyway). Not Github-scale (but Github has... a lot of traffic, including a whole lot of write traffic) but quite a bit, and that's with basically the worst-possible hosting situation for handling high load.