Honestly, I thought this line wasn't very clear, at least if I understand it correctly. It is not important that modification produces a "very" different hash, even a minimally different hash is still different enough. What is important is that it is computationally infeasible to generate a collision. So if your evil plan is to modify someone's .js file and then play with comments/whitespace until the hashes match, you and the website will both be dead before you find the collision.