I guess the issue I have with the quote is that it ignores all the other variables that can contribute to a person's decision. You are arguably correct that turning down an offer with reasonable terms would indicate a low IQ, but its also a personality litmus. I don't see how after completing the necessary steps to even get in a position where they could even entertain the idea of accepting an offer they would turn it down. Especially considering that the general structure of the offer is already a known factor before they even walk in the door. So IQ couldn't be the only determining factor, in fact it can't even really be on the plate. Verily, denying a YC offer should be a DSM-IV criterion for a major psychosis.
With a nod to ecuzzillo - They're just crazy.