entirelyuseless made the point that low cryonics use rates in the general population are evidence against the effectiveness of cryonics

No, I don't think he did. The claim that low uptake rate is evidence against the effectiveness of cryonics is nonsense on stilts. entirelyuseless' point was that if you are in a tiny minority and you don't understand why the great majority doesn't join you, your understanding of the situation is... limited.

James Miller countered by implying that this problem can be solved if one assumes that it's the elite (IQ giants, possessors of secret gnostic knowledge, etc.) which signs up for cryonics and the vast majority of the population is just too stupid to take a great deal when it sees it.

My counter-counter was that you can pick any measure by which to choose your elite (e.g. IQ) and still find that only a miniscule fraction of that elite chose cryonics -- which means that the "just ignore the stupid and look at the smart ones" argument does not work.