I have heard a lot of theories but hand size is a new one. Guys like Hakeem, Yao Ming, Dirk, Laimbeer and several other big man have been pretty good from the foul line so it is more than being big. I know Rick Barry has tried to convince every poor shooting big man to give grannie style a shot. None of them have the guts to do it in a game. His theory is that a lot of the talls guys get way to flat.The other reason the big guys don't shoot free throws is because they don't need to. Shaq was a dominate NBA player despite his FT woes. Sort of like how Jack didn't spend time on his short game until the late 70s.
There are a lot of junior golfers that sort of fall into this trap. They dominate for a while because their long game (or short game/putting) is a lot better than their peers. They don't focus enough on their weaknesses and eventually their advantages are neutralized.
I will reiterate that I think the Dan plan is hitting the fun stage. Most of us I think will agree getting down to a single digit is in the realm of feasibility for most under 40 guys if they could practice for 3k hours. The question is over the next 3k hours can you get a lot better. I am voting for 4-5 shots next year and then 1-2 over the rest of the run.
He shot a 91 in a tournament. The guy is far from where he should be... Regardless of his handicap, he needs to get more tournaments under his belt.