Any fireproof safe will have a time limit. As in how long the thing was in the middle of the fire. In most cases, the fire rating is not very long. It can be 45 minutes to 2 hours. Regular Chrome Moly steel loses temper starting at about 500 F. Wood burns at about 600 F. So, if the grips are burned, the steel, even SS, is permanently damaged too.
"...finishing processes heat the metal to 700 degrees or more..." Heat treating ad annealing, yes. However, the temperatures and time at temperature is strictly controlled. Neither is controlled in a fire.