That website you quoted is right. In general, we can think of the action of hearing as passive listening. Consider these two hypothetical situations involving a phone conversation:
I'm sorry, I didn't hear what you said. You were breaking up. Could you please repeat that?
In this scenario, it's not your fault that you couldn't hear what the person on the other end was saying. You couldn't hear him because of a technical issue.
Gosh, I'm sorry, I wasn't listening to you. I was doing something else here. I was trying to get rid of this annoying pop-up window on my screen. Could you say that one more time please?
In this situation, it is now your fault that you couldn't hear the other person. You had all the potential to listen to your interlocutor, but you weren't because you were busy doing something else.
And the same thing, of course, applies to situations when we're talking about music:
I've heard some Guns N' Roses music, but I've never listened really closely to it.