My very dumb guess is that he was going to use his thumb over the top to slide the piece out and his pointer finger drifted between the riving knife and blade? What a dummy.
Now you know how people got a major cut in their arm using a radial arm saw. Busy watching and doing something with one hand and not paying attention to the other.
That video made me cringe too. I've got a lot of respect for sharp tools after seeing how quick things go wrong. For my own safety, I keep my planer blades sharp and use proper push sticks. I actually got a good set of planer blades from that shop, and they work great.