You don't defend a quick shot like you defend an iso possession. Even the best defenders will give up a look if they stay hyper focused on their men. All it takes is a good cut to get space when you don't have to worry about dribbling.
It does seem like the zone theory is more likely than the man one, though, seeing how Green and Simmons are passing men off. If that's the case, then it definitely was Kawhi's outside rotation. If they wanted Anderson to cover the corner, he would have been on the other side of Kawhi to start. Otherwise, you are setting up the slowest player on the floor to run the longest distance (which he did and seemingly blocked Love's shot).
I do really appreciate the gif, though, as it does a much better job of showing the play than a still would. You can definitely see that Anderson didn't have Love in man. Even if he was supposed to pick him up, he was also supposed to drop him off to Kawhi and take Irving. Actually, looking at it in motion repeatedly makes it seem less like Anderson was pointing to Leonard and more like he was getting an arm up to Irving, which is something everyone else was doing on the play as well. It seems now like he was going to take Irving, saw Kawhi out of position, went like "Oh " ran to Love, and blocked his three-point attempt to seal the win.