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, ▶ this video e.g see the slide at 31:40.
in “Determination of the fifth Busy Beaver value” · View on HN