The reason folks moved to the knuckle side of the bow is finger draw. The areas where Thumb draw was most used were noted for fast shooting and smaller handier bows. When you pull with the thumb the arrow pulls off your knuckles so one needs to put the arrow onto the (to western style shooters) offside and held in place with the thumb.. The other advantage the old shooters like the Mongols and Assyrians had was the long draw thumb draw allows. A small guy can still pull into 30 inches. And they had really strong bows, and if a snap shot was needed like he is demonstrating even a short draw had considerable power. A composite like a Mongol bow starts closer to full power just strung, and that gives the short draws even more power. Too bad he got too campy making this. The shooting was enough to make his point.
That guy is amazing but the part I caught that made me pause was the statement that long distant shooting was some myth. There are actual edicts that can still be read today ordering bowmen to only practice long distant shoots and indirect fire at their regional meets so it isn't a myth.