The word “hack” is pre-internet. A “hack” journalist or a “hack job” is basically something unprofessional. It is movies that turned “hackers” into someone that gained access to the “mainframe”. In the realm of computer systems, I would argue that a “hack” is doing anything the system was not intended/designed to do. A successful DoS or DDoS needs to find some component of the system that wasn’t designed to handle the amount of traffic about to be sent to it. There are protections for DDoS, you have to figure out a way around them, that’s a hack.