This is wrong. GitHub is still a large Rails app. See presentations by the core Rails team people who work there like Eileen Uchitelle and Aaron Patterson.
That's what I am going on. It literally says the RoR stuff is just the basic webpages, everything else has been re-written or was never RoR. Same deal as twitch, the RoR is just the trivial web portion which is mostly cached. All the chat and video is in go.
This may be true of Twitch but not GitHub. GitHub uses https://github.com/libgit2/rugged to bridge between Ruby and Git. As far as I'm aware, they still use Ruby to pull the diffs etc out of Git etc.