I'm honestly not familiar enough with how well-developed graph theory is in Lean to be able to say. The paper is mostly using pretty old results, so it's mostly a matter of whether that stuff has already been formalized or not. Like anything else in software (and Lean proofs are very much software) a lot of it's about infrastructure. It wasn't so long ago that no area of mathematics outside of type theory and formal verification was really built up enough to do "serious" math -- that's changed a lot within the last few years.

What I'm more saying is that we're a ways away from being able to straightforwardly go from an LLM having a paper proof to having that proof formalized in Lean in the general case. Not so much because it's hard for LLMs, more just because it's hard in general unless all that background work has already been done. As more and more of foundational mathematics gets mechanized, it will be easier and easier to check your work in Lean while you work on the proof. For example, AFAIK unit distance has already been mechanized (though the quality of the mechanization effort sounds not great, it still greatly increases our assurance in the proof's correctness).