The "Twitter-scale Mastodon" is separate from this OP article, which seems to be a small beginner tutorial that's more of a Facebook clone tbh (it has friend requests [0]).
In any case, this title is very misleading: Rama itself is not open source [1].
[0] https://redplanetlabs.com/docs/~/tutorial6.html
[1] https://news.ycombinator.com/item?id=37138019
