Without knowing the science for sure, it seems to me that when something dies, it's likely to stop secreting stomach acid, or at least to stop producing more (it might secrete what's been produced and not yet secreted). If something croaks with a full stomach, it seems feasible that it might well not produce enough acid to digest everything. Food doesn't just fall into a huge vat of stomach acid when swallowed: The stomach produces smaller quantities of acid over a longer period. Also, I doubt it takes several days for even something as big as a mammoth to get cold enough, if not to freeze solid, to slow the digestion/decomposition process exponentially. Seeing deer carcasses my Dad hung up in cold weather suggests that to me. In sum, I need more than a bald assertion to convince me that flash freezing is needed for a corpse to retain undigested food.