In big waves you need to shift back when the nose is about to go under. But in conditions Jonathan is in, you need to avoid sinking the stern to keep the boat planing. You may need to shift back a bit and pump to get the boat planing, but then shift forward when the boat is on a plane.
Race courses rarely have reach legs anymore so speed on a reach is more of just a fun thing these days. I never thought about it, but it so windy I guess he can keep the board way up as what is left in the water is generating plenty of lift.