I think you may not be appreciating the subtlety of the repetition of the "probability of rolling a six". His claim is that aleatory probability starts with the assumption that you have a standard six-sided die with all sides weighted equally, but that epistemic uncertainty requires accounting for the uncertainty that you have a fair die, or even that it has six sides. So in both cases you are indeed trying to calculate the "probability of rolling a six", but the answers, and the process for creating them, are not the same. So while it might not have been the best phrasing for clarity, he's making a meaningful distinction.