That's always the case, though. The Giants didn't play every team in the league when they won the Super Bowl.
But every team that the Giants did not beat either a) finished with a record so bad that they did not qualify for the playoffs or b) lost in the playoffs before the Giants did.
Imagine a version of the NFL where there are only two divisions per conference, East and West.
Imagine that each team plays two games against each of its 7 division opponents,
one interdivisional game and one interconference game and two more games against basically any pro team you can find, including AFL and CFL teams.
Now imagine that instead of having playoffs, the media simply voted on which two teams should play in the Super Bowl (not necessarily one from each conference).
A narrative would quickly develop on which division was the "power" division, and because most games are divisional matches, there is no real way to dispel this narrative. This is the college football system under which the SEC has excelled.