Wind-powered vessels are very counter-intuitive: not only you can get faster than the wind by going downwind (with such a device) but you can also get faster than wind while going almost upwind (it works better when landsailing).
It's counter-intuitive because people often imagine the wind “pushing” the sail, while the force that actually happens is lift, which works with apparent wind instead of true wind.
There is still some disagreement though. Here's a more recent video by someone saying that those didn't get it right [1].
I have no idea what his qualifications are, though, as that is his first video in English, and most of his other videos don't seem to be about physics. But at least on first watch his physics sounds sound.
There is no disagreement among those that understand this thing. That video was made by someone that simply has it completely wrong. We've explained that to him in great detail, but he'll simply have none of it.
It's counter-intuitive because people often imagine the wind “pushing” the sail, while the force that actually happens is lift, which works with apparent wind instead of true wind.