I'm not sure I follow that reasoning either.
Yes, he said that if prayer was the sole motivational cause for something then it would be the most important (by definition), but I don't think that means that prayer is therefore the only motivational cause available. I think he meant more like a person who's sole motivation for something is prayer (as opposed to someone else, who isn't motivated by prayer, or not solely motivated by prayer). I'm not sure, because his statement wasn't very clear, but that's what he seems to have meant.
Of course, then you have the question of whether prayer would ever be the sole motivational cause for something. And I don't think it can be, because for that to be true, they would have to have no empathy or any other motivation to do something. It could only be provided by prayer, but if they didn't already have the motivation, then I don't see how they could acquire it by praying. Meaning that they would already have motivations, but two or more of them might be conflicting. And thus the act of praying helps to remove the conflict (whether or not they recognize that it even exists).