Running down a bunch of defensive rankings in various categories from 2017:
Opponent passer rating (very very important IMO): 7th
Opponent yards per carry: 28th
Opponent yards per play: 23rd
Opponent yards per game: 17th
Takeaways: T9-11th
Points allowed per game: 10th
Average league rank of opposing offenses: 16.3 (note: this is just about average, but some of the best ones we played, Atl (8th, 2x) and TB (9th, 2x) are teams we're real familiar with, while on the other hand we fattened our numbers against real bottom feeders like NYJ, Chi, and GB w/o Rodgers)
To me, these are the numbers that count -- against slightly below average opposition, we did a nice job against the pass, primarily because of 20 INTs, which is not to be ignored, but otherwise we were average to below average.
Now, to that "points against" stat -- yes, it's important, but you need context.
First, we had among the fewest drives to defend against, as we were only 26th in number of drives against. Now, our offense was about the same in number of drives, but was fourth in average drive time (think time of possession), and you combine this with our opponents starting with the 30th worst average field position out of 32 teams. In other words, because we protected the ball so well, and have a great punter, and so many kickoffs out of the end zone, our opponents' average offenses had far fewer chances, and had to go further, to score, than most other defenses in the league had to face. It stands to reason you're going to give up fewer points than defenses that are on the field more times per game, and with shorter fields to defend.
The most important number might be points allowed, but to really judge your defense, you have to look at it in context. For that matter, the reductio ad absurdam is you just look at your won-lost record and don't look at any stats at all.
Also, we didn't finish too strong -- 455 yards and 31 points (18 in Q4!) in a season-ending loss to the Bucs in a game we really wanted to win, 413 yards and 26 points (14 in Q4!) to the Panthers in round one, and then 403 yards and 29 points (12 in Q4!) to the Vikings, in a game we fell behind 17-0 (and really should have been blown out in, but Zimmer choked and shut down his offense).
I'll give you OPR, and points allowed, and a decent number of takeaways. But we were not in the top five in any single category, and below average in a bunch of them. Given all the above, and I don't think I am cherry-picking, I don't see how on earth anyone calls this a "very good defense" last year. They were average, and pretty beat up and fading at the end. I have no idea how this adds up to "very good."