The first 3 times I saw the replay, I thought it was a homer. It looked like Betts just closed his glove too early. I shoulda known he wouldn't do that on his own.
Then, I saw the fans had knock into his glove causing it to close. One angle shows it clearly. The other angle shows the fans hand was just barely over the imaginary line into the field of play.
Plus, watch how Betts jumps. He did not jump into the stands. When he came down, he didn't even hit the wall, except for just slightly near the bottom with his leg (maybe). His glove trajectory was not going into the stands.
The fans were right up next to the fence, and even though they had T-Rex arms, to some extent, they still went over the plane by a couple inches or so.