This is basically true, but it doesn't necessarily all have to come in one 8 hour shot. The past month I've been taking 10 minute naps once an hour from roughly an hour before lunch on through about 6 in the evening, and it makes my working hours much more focused and my after-work hours feeling much more refreshed.
I'm dubious of anything that claims to pull this off. My own failed attempts have convinced me that sleep is not something that can be lifehacked. There is some variation in sleep requirements between individuals, but you and I each have a number (probably ~8 hours) below which our bodies start getting cranky. There are loads of studies showing that any form of sleep deprivation -- and I would include here trying to optimize down your overall sleep time -- has reliable, long term negative consequences. What's more, there seems to be an unusual level of consensus on this subject. (I say 'unusual' because medical researchers never agree on anything. :-)