The 6-8hours interval is an average estimate, it's certainly not a "must" for all people.. Some people might need more, some even less, as is mentioned in the article, some people apparently need hardly any sleep at all.
I think it hugely depends on someone's everyday state of mind, thereby it being very different for different people. And it could even be different for the same person during different periods of their life. Actually I'm very sure this is the case. Consequently why could "extreme" cases hereof - people needing hardly any sleep, or on the other hand needing huge amounts of sleep - not be possible..?
Imagine naturally being in an amphetamine-like state of mind most of the time. I don't think this is impossible. I'm guessing sleep would be very overrated and pretty much a waste of time in that case. On the other hand, a person being in the exact opposite state of mind might need enormous amounts of sleep.
What I do question is whether it is possible to not sleep at all, for after a while, as far as I'm aware, people start hallucinating, falling into an extremely altered state of consciosuness. And from what I've read this state of consciousness is not very appealing to most. However, for all I know, the mechanism producing this altered state of consciousness might not come into action at all in people that need no sleep.