The D also played well against N Ill who has the 93rd offense and even that is bolstered by playing some weak teams. I believe I remember early in the N Ill game a couple of long passes against the D that should have been big gains or even td's but their qb had a lousy touch. Probably every B1G qb we have faced would have completed those passes.
I'm not one who was impressed with playing a good half of D against Colorado. Games are 4 quarters, no credit for playing well in a couple of them. Besides Colorado is terrible. After scoring 34 on NU, CU has averaged 17 ppg in their last 8 games.
So count me in the category who thinks the D hasn't been very good all year. Though I take your point Purdue isn't great with all their injuries but still managed multiple 80+ yard drives. So yes, I think talent is subpar but other teams with similar talent don't seem to be consistently confused they way NU does, which I attribute to coaching.