There is no requirement to be US born to play NFL football. About 3 percent of players are international and the NFL actually grants a roster exemption to an international player for every team. Any player that plays for an international league does so because of their inability to qualify for the NFL.
These teams are compromised of the best football players globally. So the winning team is a compilation of the best players in the world. Hence, world champions.
If the NFL gave an expansion team to London and they won a Super Bowl, none of us would have a problem calling them world champions. We wouldn’t be like …”technically they are the best team in the United Kingdom”.
Was Argentina’s World Cup championship not valid because the tournament was void of countries that didn’t field teams?