He is a heart doctor and he is talking about heart problems and their causes, so how is his expertise not relevant?
His expertise is very relevant, but it's just not broad enough for the nature of the claim. It's one thing for a cardiologist to notice an uptick in myocarditis and suspect that a recent vaccination is the cause. But correlation is not causation and making a water tight case for the latter gets into other disciplines. That's why these articles are usually authored by several individuals from various fields
Again, I'm not saying he's wrong, but there is going to be some pushback.