If Jerry Miculek says he made the shot, he made the shot. He has demonstrated many accomplishments in front of people that are "unbelievable", but unquestionable.
But the video shows the balloon popping roughly 1 to 1.5 seconds before the dent appears. Suddenly there's a spot on the white paint, but significantly after the balloon pops.
Watch the area on the plate and notice when the dent appears.
It's pretty noticeable. And the inset does not appear to be slow-motion so I don't think that explains it.