The microbes are dead but they leave behind the toxins which require greater heat to denature.
That's why there are warnings about reheating some foods over and over. There's minimal bacterial growth, then the bacteria is killed in heating sto safe temp. But the bacteria leave behind the toxins. Reheat and you get bacteria growth again before death increasing the number of toxins. Keep repeating and you have a dangerous level of toxins despite no living bacteria.