The idea is that universal prior is really about observation-predicting algorithms that agents run, and not about prediction of what will happen in the world. So, for any agent that runs a given anticipation-defining algorithm and rewards/​punishes the universal prior-based agent according to it, we have an anticipation-computing program that will obtain higher and higher probability in the universal prior-based agent.
This by the way again highlights the distinction between what will actually happen, and what a person anticipates—predictions are about capturing the concept of anticipation, an aspect of how people think, and are not about what in fact can happen.
The idea is that universal prior is really about observation-predicting algorithms that agents run, and not about prediction of what will happen in the world. So, for any agent that runs a given anticipation-defining algorithm and rewards/​punishes the universal prior-based agent according to it, we have an anticipation-computing program that will obtain higher and higher probability in the universal prior-based agent.
This by the way again highlights the distinction between what will actually happen, and what a person anticipates—predictions are about capturing the concept of anticipation, an aspect of how people think, and are not about what in fact can happen.