If it was in Lean anyone could verify it instantly. That is the huge advantage of it. Manual math Proof verification labor might be the most limited resource ever.

How does it matter if it Lean verified or a human verified proof if you comprehend neither?

There can't be too many people working in that corner of graph theory, and I expect the result to them being eminently straightforward.

One requires you to trust a human and the other requires you to trust mathematics.

Let me simplify it for the sake of argument. Imagine I am unable to follow a middle school proof of Pythagoras. How does it matter if I trust anyone beyond that? What possible contribution can I build on top of that?