You immediately self-contradicted here. If a decision heuristic often leads to the correct conclusion, then it's a good heuristic (if you are under time constraints, which we are).
Of course, given unlimited time to think about it, we would never use ad hominem reasoning and consider each and every argument fully. But there are tens of thousands of cults across the world, each insisting that they possess the Ultimate Truth, and that you can have it if you spend years studying their doctrine. Are you carefully evaluating each and every one to give them a fair shake? Of course not. Even if you wanted to, there is not enough time in a human lifespan. You must apply pattern-matching. The argument being made here isn't really an ad hominem, it's more like "The reason AI risk-ers strongly resemble cults is because they functionally are one, with the same problems, and so your pattern-matching algorithm is correct". Note that the remainder of the talk is spend backing up this assertion.
There's a good discussion of this in the linked article about "learned epistemic helplessness" (and cheers to idlewords for the cheeky rhetorical judo of using Scott Alexander and LW-y phrases like "memetic hazard" in an argument against AI risk), but what it boils down to is that our cognitive shortcuts evolved for a reason. Sometimes the reason is because of our ancestral environment and no longer applies, but that is not always true. When you focus solely on their failure cases, you miss sight of how often they get things right... like protecting you from cults with the funny robes.
> "if they look like a crank, ignore everything they say", then you miss special relativity (and later general relativity).
A lot of people did ignore Einstein until the transit of Mercury provided the first empirical evidence for relativity, and they were arguably right to do so.