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Uninformative priors are a part of Bayesian models, which are useless if they cannot be updated with real or simulated data?

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I estimate a 0.1% chance he intended it to be an uninformative prior.

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.

Maybe people you interact with do. In my world, people don't say 50% when they have no idea, they say they have no idea. Saying "no idea" might effectively be saying "uniform distribution over all probabilities" which yields 50%, but at least with people I interact with, the reverse is not true.

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.

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