Don't. If you insist on only giving advice that applies to 100% of the world you'll never say anything useful at all.

I'm sure the studies do encounter these people. But studies use statistics. These people are on the far edge of the bell curve. The mean [1] is probably around 7-9 hours. But just because the far edge of the bell curve has very few samples in it doesn't mean that those samples don't exist, nor that someone is "hiding" them.

Which reminds me: What this article does not talk about is the other side of the bell curve: The folks who need considerably more than 7-9 hours of sleep to attain peak performance and happiness. Or maybe those people don't exist, because the "bell curve" is actually a bimodal distribution, or cliff-shaped, or something. We could tell if there was any actual data here, or a link to the actual data and published science -- we could look at the shape of the distribution. But this is an article by a science journalist, so naturally it is nigh-useless.

I wonder why the article doesn't talk about the "long sleepers"? Probably because the sleep researcher quoted herein is correct: There are more people who like to believe that they only need four hours' sleep than there are actual people who thrive on less sleep. And I'm sure that no Wall Street Journal wants to learn that they might really need twelve hours sleep.

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[1] I actually have no idea if the "7-9 hour" rule of thumb is based on a mean, a median, or a mode. Pity that nobody ever links to the original research.