That certainly sounds suspicious! It may be possible to just replace inflator cartridge without replacing the entire airbag? I can't imagine any mechanic that would do that without removing the entire airbag unit from the car first. That would certainly markup the dust on your dash. I certainly would not put it past a dealership to rip off the manufacturer by doing the paperwork on recall repair to get payed for it without actually doing anything to the vehicle. I think if it were me, I would contact Mitsu corporate with all of the paperwork handy and voice my concerns.
2014 cool silver ES, acquired new 4/2015 (42.7ah at ~26K miles)
2014 Labrador Black Pearl ES, acquired new 3/2016 (41.5ah at ~15k miles)