If you click through the link on that page (and click on Compiler), there are a series of articles on the development process - step by step bootstrapping stages and some further development past that point.
This project[0] is a fork of Ben Lynn’s compiler that can be bootstrapped from a binary less than 1 KB in size. The dialect of Haskell it accepts is impressive for its size and GHC-compatible too.
It's a really interesting read:
https://crypto.stanford.edu/~blynn/compiler/
I learned about Mogensen-Scott encoding and some tricks for efficiently compiling to SKI from those articles.