Exactly!

To demonstrate in this way that the absurdity heuristic is useful, you would have to claim something like:

The ratio of false absurd claims (that you are likely to encounter) to true absurd claims (that you are likely to encounter) is much higher than the ratio of false non-absurd claims (that you are likely to encounter) to true non-absurd claims (that you are likely to encounter).

EDIT wow. I'm the person who wrote that, and i still find it hard to read it. This is one of the reasons why rationality is hard. Even when you have a good intuition for the concepts, it's still hard to express the ideas in a concrete way.