The frequentist approach is analogous to a minimax strategy in a game: no matter how malicious the universe is, you still get your 90%.

This is because you constrained the universe to only being able to present with you sequences of one of two possible values, both of which reveal themselves on first inspection 90% of the time.

Let the universe throw at you a machine that, for all you know, can produce any distribution or pattern of coins with any bias. Try to get your guaranteed 90% making predictions on the bias of those coins from a single flip.