[ctz]

The sentence is saying that collision resistance seems to be practically harder to achieve than second order preimage resistance.

One reason is a lot of hash functions are iterative in nature (you have a compression function repeatedly applied to a state, mixing the message into the state in small pieces). For collision resistance, finding a collision in just the compression function (or a limited number of applications of it) is enough to find a collision in the whole hash function (you can 'work outwards' from the collision). You can't use that effect in second order preimage resistance.

IANAC.