The pope isn't "all Catholics." The same rules don't apply to the Papacy.
You, like Mr. Johnson, seem to miss the distinction between the "sin" of heresy vs. the "crime" of heresy. The "sin" of heresy applies to all Catholics. The "crime" of heresy is nuanced when it comes to a putative pope. Pope Pius XII in Mystici Corporis is speaking about the "sin" of heresy. Pope Pius XII's teaching stands regardless of whether heresy is defined as a crime or not.