I think this is hooey. Digestion of meat is a matter of breaking up the proteins in the stomach and intestines using enzymes that clip up peptides. The protein being cooked is going to make this process easier if anything.
My recommendation is to defrost rodents in the refrigerator overnight and then put them in warm water or under a heat lamp briefly to get them up to temp before feeding. The longer a dead rodent is left at room temperature (or warmer), the more microbes in the GI tract are going to replicate, and the more foul it becomes.