"World Championships" need to be decided in competitions played by All-Star teams ("Dream Teams") from different nations, not by a league championship for club teams from cities all in one country.
You are right. In a theoretical sense, American Super Bowl winners are not “world” champions. But there is a time and a place for everything and Morgan saying this now is like telling a bride she looks fat on her wedding day. It may be true but you don’t say it.
I suppose we could set up a playoff between the NFL champion and the Canadian Football League champion. Common rules for the game could be set because the NFL and CFL have some different rules. Canada is the only other country that plays football the way the US does.
They’re the only Super Bowl Champion in the world, that makes them World Champions. And if the rest of the world doesn’t like it, they should start playing football.