It's perfectly valid as an idea, although a lot of statistical work would need to be done to show that it wasn't just happenstance and/or rule out other causes. But a link, sure.
Consider a depoliticized example: you do 30 autopsies on people who had recently been to a particular location, let's say a certain grocery store in town. Enough people go to this grocery store that it's not uncommon for way more than 30 recent visitors to be dead unexpectedly by random chance. Further, let's say in the general population, a tiny fraction of people die of a certain food-borne illness. You have a pretty good guess what that fraction is.
You do these autopsies and you find that 5 of the people died of this food-borne illness. Much higher than the proportion in the population. Now, you have a lot of additional work to do before you prove that something at the store is causing high rates of the illness: check on comorbidities, other correlated factors, plenty still to be done. But, have you established a potential link? Sure. And you picked them specifically because they died after going to this store.
Also, your idea can work too, also with appropriate statistical work, experimental design, all that. Yours is perhaps a bit better suited to answering a question like: of those who die of myocarditis, what proportion have been recently vaccinated? While a proper study the way the authors did is more like: of those who are recently vaccinated and die unexpectedly, what proportion have myocarditis? Both are valid questions and both certainly get at aspects of the link between the vaccine and myocarditis.
It's not just "pick 30 recently dead who went to the store", though.
You've picked 30 people who died after eating at the grocery store's salad bar, and used that as evidence that the salad bar is dangerous, without checking how many skipped the salad bar and died anyways.
The answer to this question would also be socially and politically relevant and warrant its own study. But that doesn't make the author's study unsound or unable to serve as evidence, like you said.
From published articles in the JAMA; vaccination related myocarditis in young men has much lower mortality and morbidity than from other causes. So were really talking about two different things here.
The second is rates of myocarditis after vaccination and the population base rate aren't the same thing either. The base rate one assumes includes cases triggered by something. So the base rate incorporates the rate of triggering events. So these rates are also not the same thing.
> Standardized autopsies were performed on 25 persons who had died unexpectedly and within 20 days after anti-SARS-CoV-2 vaccination.
This is a weird way of demonstrating a link. They picked them specifically for that, it can't be evidence of something. If they selected 25 people who died of myocarditis regardless of vaccination status or recency, and all 25 turned out to have been recently vaccinated, that would be scary data.