It's by design, up 3 scores and less than a half to go, the critical thing is to not allow quick scores. By taking away everything deep and allowing them to dink and dunk underneath (but tackling them in-bounds) you are trading yards and points for time. And they would easily have run the game out this way if not for the onside kick. Yes, they completed a few longer passes despite this coverage. Believe it or not, there are also good players on other teams we play, Philip Rivers being one of them. So it happens.
Bottom line, this is the right defense to play in that situation, and what we always used to do when we were running long winning streaks. The defense was tiring too and we'd lost some players, so I don't think we'd have had the same result if we tried to implement the same schemes they were running in the first half.
The only issue I had with the coaching was that the "hands" team should have been out there for the onside kick. I loved going for the 4th down, and always will. If I were the other teams we play, like the Ravens who meekly backed down in the same situation, it would drive me crazy.