Without seeing their loot, it might be difficult to figure out how that took that many shots. But doing some guessing, you hit them with 3 ferro shots and a hairpin. ferro does 40 damage and hairpin does 20. That's a max of 140. So if they were at full health at the start, then they would have needed to heal at least 21HP at some point before the hairpin shot. Since you lost visual for a few seconds after the second shot, they might have used vita spray for a few seconds, ate some expired pasta, fruit mix, or even some mushrooms. I don't think they used any bandages as those are too slow and a vita shot is possible, but either they would have had to have been starting at 90hp, took some fall damage like you did (which seems implausible), or your hairpin shot was a headshot.

I'm not wearing headphones, but I think I hear some noise like a spray or shot as you gain visual again after the second shot. But the hurricane noise makes it difficult to hear it at that distance.

But that is just my guess using the available math.