I think you're right. The only "secret sauce" I could see them having are some unique operational/architectural features of their back-end code that make sense

I think you're right. The only "secret sauce" I could see them having are some unique operational/architectural features of their back-end code that make sense for an entity at Github's scale. No instance of Gitea/Gitlab is even in the same ballpark wrt repo count and traffic than github.com, obviously.