.223 from an 18" bbl is 155.5db
9mm from a handgun is 159.8db
.357 magnum from a revolver is 164.3db
I don't know how they arrived at their numbers, but they can not be apples to apples. They don't say what distance the measurements were taken either.
If you've ever been to a 3 gun match, when someone switches from AR or shotgun to a handgun, the handgun sounds like a toy in comparison.