You don't need to know a universal "Proportion of True Hypotheses", that was just given as an example. But you do need to assign some probability to the hypothe

You don't need to know a universal "Proportion of True Hypotheses", that was just given as an example. But you do need to assign some probability to the hypothesis being true. Which is of course difficult to do with high certainty. But hopefully possible to argue whether it is closer to 0.1, 0.5 or 0.9 - and adjust interpretation of the pvalue accordingly.