People start with this prior in the examples you gave when they are completely uncertain and have no better mental model. It is not a good starting point for an event that has never happened and can only happen once.
Either way - let me clarify - this guy is guessing, using his brain, that it's a 50% chance. He is not saying "i don't know" or "uniform distribution" or anything of that nature. And he works in the field, so he has some insight. His guess is wildly off imo, but he isn't some clueless hack.