Sure, at the end of the day there is no reliable way to tell truth from falsehood except by thorough scientific investigation.

But the topic at hand is whether, in the absence of the time or other resources to investigate everything, there are guidelines that will do better than random chance in telling us what's promising enough to be worth how much investigation.

While the heuristic about predisposition to believe falls far short of certainty, I put it to you that it is significantly better than random chance -- that in the absence of any other way to distinguish true claims from false ones, you would do quite a bit better by using that heuristic, than by flipping a coin.