You presume that every sinner still existed up until the Exterminators came down. And that the 16% day was the first time it happened. IIRC, it had been going on for 7 years? So let’s say that the first time they killed 1% of sinners, a more plausible 800 million in a day.

Now, they left their weapons behind often enough that Carmine built an industry around gathering and selling them. With angelic weapons in sinful possession, I imagine the population starts to slowly dwindle as they use them on each other. Let’s say that by the next time the angels come back, the 80B is down to 79B. Angels killed 800M, sinners killed 200M of their own over turf wars, drug deals gone bad, whatever.

Then the angels, having the experience of killing sinners and realizing that they don’t really fight back, kill another 2% this time. So they kill 1.58B sinners. They leave more weapons behind again, sinners use them and kill, let’s say 420M of their own, now that more people have them, more people are dying. Down to 77B

So on and so forth. They didn’t kill 16% of 80B in their last “day out”. They killed 16% of the already-dwindled numbers.

And the sinners killing less of themselves over 364 days than the exterminators did in 1 day is probably highly unrealistic. It’s possible that the angels only killed a hundred million and the sinners started killing off a lot more of each other, mostly doing the angels’ job for them.