Except that’s not what he said. He said if I tell you something that contradicts the catechism don’t listen to it. And it means exactly what it says. It’s essentially what Paul told his readers in Chapter 1 of Galatians.
That’s a great point. You’re absolutely right. It is indeed essentially what Paul was saying in Galatians 1. If I or anyone, even an angel, tries to teach you a gospel other than the one that you as a Christian already know, don’t listen.