It is very possible for spayed females to produce milk and enough. It's not the uterus or the hormones caused by the uterus that cause lactate. In fact if stimulated enough a male cat can lactate. It is About stimulation of the mammary glands.
I have several intact females that have never had a litter that have in fact lactated for kittens. I do believe spayed females will lactate also. There is another rescue in my town who has spayed females they use as foster mom's that do lactate.