To me, it is as if many use atemi to make up for poor tai sabaki, timing, ma ai control and kuzushi. Iow, if they had spent the time developing these other areas of applying technique, this reliance on atemi alone for effectiveness may not be so high.
Outside of the dojo, one's chances of having poor footwork, poor timing, poor distance control and poor connection are astronomically higher than inside the dojo.
Unless I misunderstood, during your multiple attacker encounter, one guy slipped out of your kotegaeshi (poor connection ?) and you delivered atemi to the other one's face which took him off-balance ...