Quartus, you could actually figure it out for yourself if you weren't so focused on the opinions of so called "experts."
First off, let's flesh out "absolutely" a bit. A cyanide dipped BB will kill a BG -- eventually. It won't be of much help if the BG is armed, adrenaline stoked, and charging you from 7 yards, though. The context of the premise is that "absolutely" means "immediately" as in "drop the charging BG within 21 feet."
Here are a few things for you to think about:
1. Any hit is better than a miss, even though there are very few places you can hit a BG with a handgun that will "absolutely" stop him. Sure, a COM hit with a small gun eventually causes the BG to die in many cases, and that happens way too often with large guns as well.
2. The only places you can hit a BG to effectively ensure he's "absolutely" stopped are in the upper spinal column, brain stem, or brain. The problem in hitting the spinal column with a COM shot is all the stuff in front of it like clothes, fat, muscle, organs, etc., that tend to deflect and slow down the bullet so even if the flight trajectory was perfect, the bullet may not have any effect on the spinal column.
3. "Expert" instructors teach COM shooting because (a) almost anybody can hit a BG COM from 7 yards, (b) any hit is better than a miss, and (c) the student develops more confidence in his skills under that syllabus.
4. If the actual "absolutely" stopping areas were marked on the target and the instructor taught that if one of them is not hit, the student would be dead, then the student would get another instructor so he could feel better about his inadequate skills.
5. It's a good idea when relying on any gun to think that you're using a single shot weapon. The odds of getting a second shot are considerably less than the first for MANY reasons that have nothing to do with the gun's reliability. They include the BG's attack, second shot recovery, and a host of other things. Regarding semi-autos, has ANYBODY had a jam on the first round that's already in the pipe...?