[p1mrx]

It should be straightforward to design a hash function that can't be preimage attacked using all the computing power in the universe.

Maybe an omnipotent cryptographer would find flaws in SHA-256, but then they could design a better function and include it in the book.

[kzrdude]

Maybe their omnipotence includes being lucky enough

[p1mrx]

Why would an omnipotent being need luck? They have infinite brute force.