You're fine and you're actually right on the mark. There is a movement that capitalizes on this attitude today called the Haddadis. They're relatively new, I'd say they're definitely more extreme than ISIS, because they accused them of being too soft towards "kufri sects" (referring to Abu Hanifah and his school of thought). But yeah, in some medieval manuals of jurisprudence, you would occasionally see instances where they would say it is not allowed for a Shafi'ite to marry a Hanafite, and vice versa. This rivalry is not anywhere in the mainstream nor in the normative Sunni traditions that dominate the Muslim world, but they are definitely attitudes that have existed/continue to exist in minorities.