If his left knee was down, then it was not a TD, but if it remained up, I believe it was a TD.
As I cannot see the left knee because of the angle with the bodies in the way, I would not reverse the call on the field. In addition, the angle is bad from a depth perception perspective too. It is not right down the line so he may look like he hits the ground with the ball just breaking the plane but it may not be the case. With that said, in the angle we have, I don't see evidence he was down prior to crossing the goal line.
Just shows how tough it is to officiate some of these plays. They do a horrible job but that doesn't mean it isn't a tough job to get right.
FWIW, I am glad he was called short because they should have been taking knees and hawaii players should have been going at Webb's knees the last couple of plays. The old Hawaii teams would have been doing that all game, let alone in that spot.