It doesn't "mean" that the a priori is 50%, I guess what you want to say is that if we consider the a priori probabilities are equal between the two hypothesis,

It doesn't "mean" that the a priori is 50%, I guess what you want to say is that if we consider the a priori probabilities are equal between the two hypothesis, then we can use the 5% p-value. But if the probability of the hypothesis is much less likely, we might need a p-value much lower to be sure. Edit: my comment doesn't mean much since you edited your first sentence.