There is a rule, but it comes into play only when the follow through hinders the catcher's attempt to make a play--with no runners on base and less than two strikes it wouldn't apply; it also wouldn't apply on a foul.
In those situations where it doesn't apply, saying something as this umpire did falls under the category of preventive officiating. It's appropriate to say something even though the play that just happened doesn't warrant an interference call. Not required. Often helpful. It is widely recognized as an effective practice.
Similar examples would be when a runner makes an illegal slide on a force play that doesn't interfere with the fielder but would have if he had got there sooner. If I'm the plate umpire who has come out to the mound to observe the slide while the base umpire covers the throw to first, I may have nothing on that play, but the runner needs to know what he did could cause problems, and I'll tell him as he trots back to his dugout.