I don't know if hustle is enough for success, but I don't think people simply mean "working hard at doing all the things that make a company successful." when they use the word. Even though the precise definition is hard to pin down, you can easily spot it during its absence.
There are times when you wish to do something, and you avoid it because it just feels like you might be pushing it a bit. If you still did it anyway - what you did was hustle. In fact, I would say that if you reflect on your life and think of the times when you didn't take that invitation to an event, or that break to go on a vacation, or introduce yourself to a person who you liked, and then realized that the choice you made was too safe, and your choice (lack of hustle) didn't help you all that much long term anyway, you can see its absence quite plainly.
Having said that, I also agree with you on one front. Instead of using the word hustle, if these articles gave an example, and then maybe made a point about why that is hustle, it would read much better.