This seems to be the norm for the electronic types, I would be very surprised if the ones at this range did not work the same way.
I really doubt that it's the muffs. I'm not even sure that an always-on amplifying muff would have this effect, because even they have an upper threshold. What may have happened is that she didn't place the muff over her ears properly. I've seen women try to wear muffs over their hair, for example. It seems unlikely, but two older women disoriented by their first time at the range - entirely possible.
Also I'd be very surprised if anyone actually handed them a loaded gun and had them enter the range.
The fact that Justin Bieber is their attorney causes me to suspect that their claims may lack merit.