Let’s say that there’s no infield fly rule, then if the ball flies on infield, a defensive player can catch it and force the hitter out, but instead of doing that he can intentionally drop the ball, which would force the 1st and 2nd player to move to the next base(which would be pretty hard if they were standing on their own base), and it would be way easier for the defense to achieve double play(or even triple play depending on the situation), because they can just pass the ball to each base and get multiple outs. So to prevent this, when the 1st and 2nd base is full(3rd base could or could not be full doesn’t matter) and the ball flies on infield, umpires can call ‘infield fly’ under their own judgment. Then, the hitter would be out no matter what even if the defense drops the ball. Normally, if the defense drops or misses the fly ball, all base is forced to progress to the next base but when infield fly is called, even if the defense drops the ball, other runners aren’t forced to run to the next base. So in the clip where the infield fly was called and the runner came in to the home plate and scored, the runner wasn’t forced to progress to the next base. So when the defensive players stepped on the home plate, it didn’t do anything because the runner wasn’t forced to get to the home plate. The only way to get the runner out is to tag him, but defensive players didn’t know the rule and they thought the runner was out when they stepped on the home plate. And that’s how the runner scored

@@string1641 this was very helpful. Thank you!

Remember the infield fly rule is effect. Automatic OUT!!!