You don't have to trust the full Lean toolchain, only the trusted proof kernel (which is small enough to be understood by a human) and any desugarings involved in converting the theorem statement from high-level Lean to whatever the proof kernel accepts (these should also be simple enough). The proof itself can be checked automatically.
Lets assume the kenerl is correct and a mathematian trusts it, now what happens if it is the proof itself that he cannot understand anymore because it has gotten too sophisticated? I think that this would indeed be a difference, and a crossing point in history of mathematical proofs.
In principle, it's no different than failing to understand the details of any calculation. If the calculating process is executed correctly, you can still trust the outcome.
