It's not that black and white. The DDOS attack itself is not a hack but to run a successful attack you need a good amount of preparation, knowledge and lots of reconnaissance. Things that are usually considered part of a "hack".
"In the computer security context, a hacker is someone who seeks and exploits weaknesses in a computer system or computer network."
DDOS can be done in a number of different ways. It's not just sending as many bits as possible over a line. For example you can run a DDOS by breaching the frontend and making the servers you are attacking working for you by duplicating the streams. Read up on it a bit. running a DDOS is not done by simply clicking a button. There are just too many different angles to attack from.
The next thing to consider is that availability is a part of network security. Meaning if you successfully compromise the availability of a system you compromised security.
Last but not least for running a DDOS you need a large enough botnet. How do you think botnets are created?
The thing we have to actually consider is not if an attack can be defined as a hack but if an attacker can be defined as a hacker or a mere script kiddie.
If you demand respect or gratitude for your volunteer work, you're doing volunteering wrong.
