Isn't DDoS made in the mempool only? Essentially, an attacker broadcasts dust transactions to occupy the space and prevent from other transactions to be mined. I suppose that it doesn't matter if the block size is 1MB or 1GB, because the analogy of the median fee and the total fees the attacker has to pay remain the same.

It is surely not a “hack”. Maybe “attacking” suits more properly.

Generally a hacking is when someone explores a system weakness. For example, the person who found the leak in a bunch of lines from Poly's source code is a hacker.