As someone stated above this is probably just a redbull commercial, not really a race, and we are not really the target audience. But still it leads to some interesting questions about speed.
"straight down wind" ? humm.. apparent wind wise the AC72 is close reach to clause hauled, true wind can not be straight behind. I watched the video again, it's hard to tell what the real wind angle is though. But it looks like Kai is using a big kite, so wind is probably say 20 kts, and he might be going say 25 kts which is already quite fast for a surf board on chop.
Those AC72 can go like 3 times the wind speed (in light winds of course, there's an upper limit) and I believe kites only go slightly faster than the wind, someone correct me if I'm wrong. So it seems the only chance for a kite against a race boat is in very strong winds on very flat water... Unless Oracle spares 0.1% of their budget for a true race kite as someone else suggested