AROTC already mentioned it earlier, but heating the retort pouches in boiling water and then drinking that water is not a problem. I believe that you're thinking of the flameless heater pouches where you add water and an exothermic reaction heats up your ration pouch.

I believe that the main reason why you shouldn't drink the water from a used flameless heater is because it will act like a laxative, which could then dehydrate you rather quickly due to diarrhea. The water you add to the heater reacts with dry magnesium-iron powder inside to generate heat. The solution you end up with contains, among other things, magnesium hydroxide, which is what Milk of Magnesia is.