I take exception to the characterization of "a few opportunistic plays" in a way that makes them sound like luck.
One or two plays might be luck; consistently winning the turnover battle is coaching. More than a few defenses of top ten teams this year had a strategy of "bend but don't break" and exploiting and creating opponent mistakes. I forget the team; but one coach (Oregon?) went so far as to say they had to learn to celebrate field goals.
If you can keep the other team out of the end zone and get some turnovers then that is a valid winning strategy regardless of yardage. You have to shape your strategy to the talent available and which positions are strongest. Sometimes that means an aggressive shutdown defense that risks giving up big plays in favor of getting of the field quickly. Sometimes that means the often hated but sometimes effective "bend but don't break" strategy.
I think we made the most of the talent we had this year on defense. And, more than any particular schematic strategy, we exhibited remarkable leadership and fight on defense.
More than once, and more than one player, didn't give up on plays that seemed lost. Keeping huge plays out of the end zone turned touchdowns into field goals, or better yet, fumbles. And since it came from a few different players, I credit coaching to the motivation to never give up on a play.
I also saw improvement in the second half of most games. That's one thing I always loved about CPJ. No matter how badly we played in a first half, our offense seemed to make adjustments better than the other side. While the overall performance on the D was WAY below the offense, I did see improvement in the second half of most games.
I give credit to the players who made those remarkable plays (not merely "opportunistic") but I also credit the coaches for starting the players with the heart to make those plays, over sometimes more physically talented players, and motivating them.
Having said all that; yeah I would like the confidence that the D could shut down an opponent when needed. But let's give credit where it is due. And in the second half of the season we made enough "opportunistic plays" against different opponents that I can't attribute them to dumb luck anymore.