That was my thought as well. Garbage in = garbage out, that's pretty standard in most fields. I really didn't like how the author treated the theorem as if it's some sort of magic, aside from something everyone that's taken a college prob/stat class has derived from first principles.
The problem is that advocates do treat it as a magical thing. They extrapolate from the fact it is proven to the claim that all knowledge is Bayesian, to the implication that all Bayesian reasoning is knowledge.
This fashion is why, for example, BT has been used to both prove the resurrection of Jesus, and to prove that Jesus didn't exist: both to a very high probability.
I'm almost sad I've never met one of these people in the wild. I'd really like to sit down and watch someone, with a straight face, try to say they both have a set probability for their belief on Jesus AND the probability that some vague ~evidence~ would exist given that belief.
