^^ So it's all about size. Ok, so if China started playing American football and developed a league that was bigger than what America has the winner of their NFL league can call themselves World champions even if they have never played anyone outside China. If you can do it so can they, right? :2 cents:
The size matters because the NFL is the
only American football league. If China somehow created a larger league, then they would be in the same deal as soccer in Europe - they would have to play the Americans because there is another league that is comparable to them in skill level. As it stands now the list of world class American football leagues begins and ends with the National Football League; it represents the highest level of American football. Hence, they are the best in the world at American football. Another term for that is world champions.
It is not the same as, say the National Hockey League, which is the highest level for hockey. In hockey's case, there are several high caliber leagues in Europe - most notably the KHL - which could challenge the NHL on the ice. In American football's case, it is played in one country (With annual games in Toronto and London), and it is the only professional league of its kind. There is no other professional American football team, and if there is, the NFL is so massive and well known it attracts all the best players, leaving the other leagues with second-rate players. This is sort of what happened with leagues like the XFL (Of course, the XFL just sucked entirely, so that's a bad example). The only other notable professional football league in North America is the Canadian Football League, which plays another version of the game.
Being the only pro league for American football, as well as the largest sports league of any kind in the world, it makes sense that the Saints can call themselves the world champions; they won the highest competition of their sport. By your logic, the winner of the FIFA World Cup is not a world champion, because there are comeptitive teams in the world that do not get a chance to play in the World Cup.
Is it a little stuck up to call themselves the world champions? Yes. That doesn't make it untrue, though.