Possible Mooney Trade?

I don't think it is a rule, more of a trend.  If you look at the makeup of all Superbowl winners over time, you see an obvious shift from the long traditional team built on a strangling defense combined with an unstoppable running game with a decent to very good passing game.  It started when all the offense-oriented rule changes came down starting in 1974 and with a real boost in 2002 and beyond (link). 

The example you use occured before the steady changes of the NFL rules started in 2002 and continues for the next 10 - 15 years that gave decided advantage to team offenses over the defense.

If you look at the offensive and defensive rankings of all SB winners over the last 20 years or so, you will see that the winners have mostly moved to having a top 5 - 8 offense.  Over the last 10 years, only one SB winner was not in the top 10 offensively and only 3 were not in the top 5.

You do see teams with an offense in the top 10 winning a SB (Eagles in the 2017 SB).  But it is occurring with less and less frequency.  Unless we see a major rule change, I expect it will continue.  And you are seeing teams with a defense not in the top 10 winning the SB more often (KC last year).

I am not arguing that having a strong defense is a detriment, just that having a strong offense (and an adequate passing offense) is of a greater benefit.