Raging Thunderbolt educates:

The fallacy that Sully commits is called denying the antecedent. As Wikipedia, everyone’s go-to logician, says:

One way to demonstrate the invalidity of this argument form is with a counterexample with true premises but an obviously false conclusion. For example:

• If Queen Elizabeth is an American citizen, then she is a human being.
• Queen Elizabeth is not an American citizen.
• Therefore, Queen Elizabeth is not a human being.