You are correct. That is exactly why this attack cannot be used to forge documents (at least in a direct way)! To forge a document, you need to find another d

You are correct. That is exactly why this attack cannot be used to forge documents (at least in a direct way)! To forge a document, you need to find another document having the same hash. So it is a preimage attack. Using a collision attack you can find collisions in the hash space, but that is not useful directly.