... just making the point.
A rat is someone who reveals the truth about a crook's secrets.
If the crook calls someone a rat, he is in a position to know whether the rat is telling the truth. By calling the rat a rat he acknowledges that the rat is revealing his own secrets. Thus Trump calling Cohen a rat is an admission of guilt.
If the president admits his own criminal actions in the campaign to become president, why is he still president?