The short answer is that I wrote both the proofs and the language the proofs are written in myself, so it might be difficult to get up to speed on it as a third party. One resource I can point to to learn the metaprogramming language is:
which goes over the lisp language syntax and builtin functions. There is also a tutorial here:
https://www.youtube.com/watch?v=A7WfrW7-ifw which shows how proofs can be written, although it doesn't go in too much detail on the metaprogramming language. The file is written more or less as shown in the video, using a combination of writing proofs from the outside in with `_` at all the unfinished positions, as well as (focus) blocks once the proof becomes large enough to be difficult to work with. Metaprogramming inside proofs is very rare, but sometimes you will see variable declarations or functions inside a proof when it has a repetitive structure.