I think this just has to be the problem statement, there's several lemmas I would expect to see in there. Granted I know very little about Lean but it seems like the question and not the proof outlined in the paper.
They are using a Lean tool where you separately state your theorems with `sorry` and then prove them elsewhere. The tool checks that all sorry's are covered. This is so the AI doesn't need to edit the specification of the theorem statement.