Correct as in the Lean proof correctly finds a counterexample to N-S.
Here is what putting trust into a Lean proof means https://ammkrn.github.io/type_checking_in_lean4/trust/trust....
In particular the main ones are:
1. The theorem has been written correctly.
2. No exploit of a kernel soundness bug in Lean (and the independent kernel nanodo, which OpenAI also checks against).
In particular, if you trust this, then you don't need to care about anything else the Lean program does, no matter how many lines, lemmas etc. it makes along the way.
As I said above, the theorem statement have been written independently by formal conjectures, and you are free to read it yourself (or trust other people have done it).
So, assuming you don't disagree the theorem statement have been written correctly, you pretty much need to believe 2 is false, if you don't trust the proof [1]. And that's what I find has a rather low probability personally.
[1] As the book says, you also need to trust the hardware, firmware etc.