That's a great counter-example and a reminder to me that I shouldn't use make universal statements that include "always" or "never".
In one area, the most important, it was a dramatic improvement as you say. The most important thing that Quinn brought was turnovers. The defense didn't have to dramatically impove the amount of yardage given up, that took a little longer. That's because the turnovers not only gave their offense more opportunity, it subtracted opportunity from the opponent.
Any time you see a defense ranked much higher in points allowed than in yardage, it's typically a sign that the defense is forcing a large amount of turnovers. However, what happens if the turnovers don't come?
If I look at points allowed you are absolutely correct. If you look at yardage, particularly against the run, you can see that it took a few years to get better, although there was no dip when Quinn was hired. So, I can't wiggle out of this one.