Yeah exactly. Mathematicians have been happily developing Lean for the last decade intending to use it on the sort of math problem where the solution would be extremely long and tedious for humans to check manually. Turns out automatic correctness checking was exactly what AI needs.They have LEAN to help them, a maths programming language of sorts that essentially allows them to check if the logic is correct.
It sounds like as @EndME says basically this person had Claude write the proof in Lean:
The Lean sources in this repository — definitions, statements and proofs, together with Challenge.lean, Solution.lean and the metadata — were written by an AI system (Anthropic's Claude models) working autonomously under the direction of Justin Leder; no human wrote or edited the Lean code. Correctness rests on mechanical checking [...] The work has not yet been refereed by human mathematicians or by anyone independent of the author; the only review so far was carried out by AI systems (adversarial reads of the formal statements and of the proof chain against the cited literature). Mechanical checking does not cover whether the formal statements express the intended mathematics: readers should satisfy themselves that the definitions in Challenge.lean state "θ(p_c) = 0 for nearest-neighbour Bernoulli bond percolation on ℤ^d, d ≥ 2".