No, the OP is exactly right and saying this is a myth is just something fashionable to say nowadays without any proof to back it up.
You never asked for any! I have plenty and would be happy to to show you. You just jump on the cliche bandwagon. Go back 1 year, remember that mighty SF defense? 2nd best defense in the nfl and 11th best offense when they played the ravens with the #10 offense and the #12th best defense. With a superior Defense the 9ers should have dominated if you believe in that cliche.
Lets go back the previous year. Giants with a #9 offense and a #25 defense (YES 25TH RANKED), beat NE who had a better offense and defense (#3 and #15). How the hell did two teams without a top 10 defense make it to the SB?!?!? Thats impossible! (please note sarcasm)
Then there was that 2010 season. The Packers went up against the best defense in the league in the SB and got stomped. Thats how it went right? No? Oh right Packers won with their 10th ranked offense.
Back another year we had the colts and the saints. Saints came in with the #1 offense and the #20 defense facing the 7th rated D and the 8th ranked O. Team with the #1 offense and 20th ranked Defense won.
2008, well here you go, the team with the #1 defense won. We found 1 example!
2007 1st ranked offense and 4th ranked defense of the undefeated patriots faced the Giants who had a 14th ranked offense and 17th ranked defense. The team with the worse offense and worse defense won (hint great matchup for Giants).
Should I continue?
the number of Super Bowl champs with a top 10 offense? Thirty-eight. And a top 3 offense? Twenty.
There have been 427 NFL playoff games over the last 45 seasons. The better defensive teams have won 58 percent of them. The better offensive teams have won 62 percent of the time.
Among the 45 NFL Super Bowls, the better defensive team — measured by points allowed that season— has won 29 times. The better offensive team won 25 times. (Note that adds up to 53, which means that some teams are the better offensive
and defensive team in the Super Bowl. Nineteen Super Bowls have featured a team superior on both sides of the ball. Those teams have won 14 of those games.)
Credit -
http://freakonomics.com/2012/01/20/does-defense-really-win-championships/ for those last 3 tidbits