All the passage says is that if you believe the coin is unbiased, then you expect to see a roughly 50-50 split between heads and tails. If you expect to see 70:30 split of heads:tails, you ought to believe that the coin is so biased before you do the experiment. It looks trivial when applied to coins, but less so in other contexts. This is a statement about priors, not posteriors, hence the term "expectation". In Eliezer's example, if you are p% confident that an accused is a witch, then you should expect a definitive witch test to exonerate the accused (100-p)% of the time. If any outcome "confirms witchiness", then the test in question is not a test of witchiness.