[meiji163]

In Theorem 1.1, f is a function of random variables, which might be where you're confused.

> doesn't that mean y is a predictable function of x

Sort of: as function of real numbers, sha256 is just some deterministic function. But point is its output "looks like" a uniform random variable for any reasonable input distribution i.e. as a function of random variables the input and output variables should have 0 correlation