Exploit. The system worked as intended, just without a rate limit. A hack would be relying on a vulnerability in the software to make it not function as programmed.
It’s the difference between finding a angle in a game world that causes your character to climb steeper than it should, vs rewriting memory locations to no-clip through everything. One causes the system to act in a way that it otherwise wouldn’t (SQL injections, etc) – the other, is using the system exactly as it was programmed.
Downloading videos from YouTube isn’t “Hacking” YouTube. Even though it’s using the API in a way it wasn’t intended. Right-clicking a webpage and viewing the source code isn’t hacking - even if the website you’re looking at doesn’t want you looking at the source.