Formalizing proofs in Lean has gotten dramatically easier since the formalizations available in 2005. And Lean’s mathlib has done most of the underlying work so that you have its axioms and necessary lemmas baked in. You can think in terms of standard abstractions that look very much like the exact notation in the undergrad textbook.

That said, I am not in any way trying to discount how incredible of an achievement it is to formalize a millennium prize winning algorithm in Lean. I mean just look at the code that OpenAI published. It’s like an encyclopedia of different fluid dynamics concepts.