I’m a Lions fan and a post in r/detroitlions about some knockoff merchandise that called the Lions “back-to-back Super Bowl Champs” had me wonder this. The Lions did win back-to-back NFL Championships, but those were obviously pre-Super Bowl.
The Vikings are interesting though - they were NFL Champions in 1969 in that weird few years after the Super Bowl became a thing but before the merger happened. So the NFL Champion Vikings played the AFL Champion Chiefs in Super Bowl IV and lost. Technically the Vikings did win the NFL Championship that year though.
In 1969 the NFL and AFL were not one league they were two separate leagues that did not play games against each other other than an exhibition at the end of the season. NFL teams also played exhibition games against college teams and Canadian Football teams. The SB was an exhibition game it was meaningless…