That was the time of the first quotation of a divine affirmative statement in scripture about it; however the concept of animal sacrifices was much older (it was the point of friction between Cain and Abel), and the use of skins was obviously deemed okay. It would be surprising if in such a situation people yet remained vegetarians.
I’d want to know how the whiz kids deduced this. Humans and humanoid animals have traditionally been omnivores.
The use of skins was deemed okay because God Himself first made coverings for Adam and Eve out of an animal skin.
However the sacrifices were burnt offerings and they were consumed by the fire. Nowhere in the sacrifice rituals did anyone ever eat the animal after it was made a burnt offering to God.
Permission was given only after the flood, after the entire earth’s ecosystme had drastically changed. And at this time God put fear of man into animals because of this change, so that they wouldn’t be hunted to extinction.