An interesting thing about this is that they figured out a high-level description of what this Busy Beaver program is doing - it's computing a Collatz-like sequence until it terminates.
I'm not sure if that is described in this paper, but I learned about it in this Scott Aaronson talk,
https://www.youtube.com/watch?v=VplMHWSZf5c
e.g see the slide at 31:40.