I can only relate with my own experience with being hit with UDAP in the face; it takes a very long time to wear off. Even with a water source (garden hose, full blast in my face for instance) can take twenty minutes for it to subside to the point where I could function again. My encounters with black bears tell me that at that point, the last thing they are going to do is want another encounter.
I also got this from the article
" attack with a shotgun blast directed over a bear’s head may very well never get reported, he said."
It should be noted this happens at least a dozen times a day every day in the summer here. All the float plane pilots I know exclusively use 12 gauges with rifled slugs, and while they fire them often, I only know one that ever shot directly at a bear (he missed, bear ran away anyway!)
If it was more specific I'd guess that the vast majority of handgun uses were similar in that they were scaring bears. If a Brown or Grizzly bear are setting on stealing your picnic basket a 9mm is joke, you are much better off with bear spray imo.
As for the carrying multiple cans....perhaps on a long hike without any resupply points or communities nearby I suppose, but emptying a can of spray on a single bear is INSANE. The stuff doesn't compound like that anyway, and there is defiantly a delay in it kicking in to disable your eyes/throat/nose, so as bad as it sounds when you are in that kind of situation, it is really important that your first shot is good. Also, on most brands, you can't just tap it, once you hit the release it continues for several seconds no matter what you do as a design feature.