Ah. Your use of not true made it look like an outright dismissal of his whole statement. As for the order of when to pick the prior, I think what is more important is that the data not influence your choice of prior. If you were some oracular machine you could see the data and generate hypothesis and priors for them independent of the data and still not fall for the problem you state.

And then there is the problem of how do you form sensible hypotheses without at least knowing the shape of the data first. The form of these hypotheses are themselves a restriction on the possible space. I think that is what the GGP was getting at.