Pretty uninformed statement. There are so many variables that go into a quick turnaround. For instance who did they play? Most of the teams they played after the change were run heavy teams. In the case of Wiscy it was a run heavy team with a depleted OLine. (See
@Middle-aged_Ball_Coach posts on the lack of depth for Wiscy Oline) Then they beat Ga Tech. another run heavy team. Stopping the run is often about discipline more than talent. So were they really better then when they played NE/IL or was it really about match ups? It could be as simple as the players really did not like the previous DC. Thus they started playing with more effort once he was gone. Only Purdue is an outlier as a non running team. They had no film to watch of the D with the new coordinator. So maybe they changed schemes which confused the Purdue HC. So we will see now that teams have film if he truly is great or whether it was a flash in a pan.
Modern defenses can be on par with rocket science. The old adage that defense is only about effort is simply a wives tale. Now maybe this new DC really is a magician that waves his magic wand and the D is magically better. I would say that the sample size is too small to make that determination.