There is no direct correlation between a productive passing offense and a good defense.
Boise St: #6 Passing Offense; #2 Scoring Defense
Alabama: #27 Passing Offense; #3 Scoring Defense
Stanford: #29 Passing Offense; #10 Scoring Defense
Missouri: #33 Passing Offense; #6 Scoring Defense
You can look back through the years and find plenty of teams that pass well and have solid defenses. Because a team is highly rated at passing does not automatically mean they can't play defense. There's no evidence to support the allegation that you can't do both.
Actually, what you will find is those teams rushed the ball well. Boise State was #26 in rushing offense, Bama was #36 in rushing offense, Stanford was #18 in rushing offense, and Mizzou was #49 in rushing offense. These teams were balanced teams that could pass the ball but could also run the ball if need be. In 2009, Texas Tech run offense ranked #113. In 2008, TT run offense ranked #96. In 2007, TT run offense ranked #120. In 2006, TT run offense ranked #112.