Hydrolocking is definitely bad news, but what you were experiencing was simply from moisture hitting the MAF sensor, which will make the car run rough and sometimes stall. Spray from the car in front of you coming through the empty fog light hole can definitely cause the MAF sensor to get damp and not read properly, but the dealership was just trying to scare you into spending $1k in their service dept. Rain hitting the filter cannot cause hydrolock.
15.07 @ 93.49
Team Duratec #149