With [the universal] prior, TSUF-like utility functions aren't going to dominate the set of utility functions consistent with the person's behavior

How do you know this? If that's true, it can only be true by being a mathematical theorem...

No, it's true in the same sense that the statement "I have hands" is true. That is, it's an informal empirical statement about the world. People can be vaguely understood as having purposeful behavior. When you put them in strange situations, this breaks down a bit and if you wish to understand them as having purposeful behavior you have to contrive the utility function a bit, but for the most part people do things for a comprehensible purpose. If TSUF's were the simplest utility functions that described humans, then human behavior would be random, which is isn't. Thus the simplest utility functions that describe humans aren't going to be TSUF-like.