Not having knowledge of x is just one type of preimage attack, a "preimage resistance". You can also know x, then it's a "second-preimage resistance".[1]

Which is not the same as a collision attack. Where you're trying to find x and y such that h(x) = h(y) without anyone specifying x or y in advance.

By definition a collision attack is an attack where you specifically craft both x and y such that they exploit weaknesses in the algorithm.

It's not enough to know an arbitrary x or y that someone else has made, because that value isn't going to be exploiting the weakness.

1. https://en.wikipedia.org/wiki/Preimage_attack