I don't disagree that the D is genuinely good. However, part of the reason they held purdue to much lower than their offensive averages is because the O stayed on the field allowing the D the legs, some luxury to be creative, and face fewer opposition drives. I will concede that they might have been able to maitain the per play average for even more drives but the more chances you give pu (or anybody for that matter) the higher the likelihood of giving up points.
Even against osu and mi, the D was indeed good but the final score wasn't because the O did nothing (neither via scores nor TOP) ie they left the D was out to dry. If the D had to face fewer drives and/or enjoyed better field position, the osu and mi scores (for that matter even il) don't end up where they did.