Well, it only applies to balls hit right near the top of the fence. And the fan likely didn't change anything ultimately because it looked like Betts was going to catch that. It could have been jarred out of his glove once he hit the fence if the fan didn't touch his glove, but he would have at least had it in his glove initially.
And it looked like the right call to me. The fans were reaching over the top of the fence and Betts wasn't at the wall yet, so it appeared that the ball was over the playing field.