Here’s the thing: Does Tom Riker actually prove that? That’s the explanation suggested in the episode, but the preponderance of information about the mechanisms of transporter technology, as given both before and after, conflicts with it. But there’s another hypothesis, a simpler one, and one that we know for a fact transporters are capable of, because it’s a recurring element in Star Trek: Thomas Riker is from another universe, brought to the Prime universe by similar means as many of the various visits to and from the Mirror universe.