If it was so clear, don't you think others would have seen that? I didn't.
The video that shows him hopping was one from a replay of the gate-breaking incident which was shown 5-10 minutes after the race on NBC.
I'm not a vet but I watched the horse going back to the gate and I saw no signs of lameness, much less the horse being three legged lame. I think the consipiracists (if there is such a word), just love to eat this stuff up.
Not even the announcers noticed it, but if you are watching the leg, you can't miss it.