I'm familiar with the guys channel. He has made high powered slingshots and ones that shoot arrows as well. From the guy and also looking at the video, I don't believe it was a hoax, just a bad ricochet. I have had my share of ricochets before over the years.
One was from a slingshot, one from a bb gun rifle, and the other from a .22LR
The bb gun one bounced off the backstop (plywood wrapped in vinyl) and hit me in the face about 20 yards back. This was nothing more of a humorous happening and did no damage.
The slingshot one hit me in the arm, carried more energy and left a welt, also from about 20 yards.
The .22LR one was from about 40 yards and hit my target (tin can) changed direction in the can, exited at a different angle, hit a rock, and bounce back towards us, grazing my friend's arm and drawing blood, but not worse than a minor cut. Probably because it was tumbling and already deformed with sharp edges from hitting the can and the rock already.
I'm guessing that the direct force of the slingslot ball bearing he used, combined with the fact it was most likely no longer perfectly round and smooth after ricocheting is what caused the bleeding.
If a $10 wrist rocket from Wal-Mart with 6mm ball bearings can leave a welt on my arm from a ricochet, I'd say a powerful custom slingshot with 20mm ball bearings could probably hit hard enough to do that damage on ricochet.