There are really two claims here. The first one -- that if some guy on the Internet has a model predicting X with 99.99% certainty, then you should assign less probability to X, absent other evidence -- seems interesting, but relatively easy to accept. I'm pretty sure I've been reasoning this way in the past.

The second claim is exactly the same, but applied to oneself. "If I have come up with an argument that predicts X with 99.99% certainty, I should be less than 99.99% certain of X." This is not something that people do by default. I doubt that I do it unless prompted. Great post!

Stylistic nitpick, though: things like "999,999,999 in a billion" are tricky to parse, especially when compared to "999,999,999,999 in a trillion" (which I initially read as approximately 1 in 1000 before counting the 9s) or "1/1 billion". Counting the 9s is part of the problem, the other that the numerator is a number and the denominator is a word. What's wrong with writing 99.99% and 99.9999%? These are different from the original values in the post, but still carry the argument, and are easier to read.