Objectively false.
Teams that have won a title being ranked 10th or worse on offense (since the NBA/ABA merger):
'10 Lakers (11th)
'08 Celtics (10th)
'04 Pistons (18th)
'99 Spurs (11th)
'94 Rockets (15th)
'90 Pistons (11th)
'79 Sonics (14th)
'78 Bullets (10th)
Teams that have won a title being ranked 10th or worse on defense (since the NBA/ABA merger):
'01 Lakers (21st)
'95 Rockets (12th)
'82 Lakers (10th)
Since 2003:
Average offensive ranking of NBA champion: 8.08
Average defensive ranking of NBA champion: 4.75
Draw your own conclusions.
In 2014:
Heat #2 offense, #11 defense
Spurs #6 offense, #4 defense
The Golden State Warriors and Cleveland Cavaliers were consistently the two best defensive teams in the NBA throughout the playoffs. In addition to having LeBron James, the reason the Cavs were able to keep that series against GSW so competitive even with their injuries was because they played better defense on the Warriors than anybody else. Even so, the Warriors probably don't win that series without playing better defense on Cleveland than anybody else had thus far as well - Iguodala in particular played fantastic defense on LeBron.