I asked John Walton and Tremper Longman about this, and they both agreed that there’s just not enough in the text to make a strong statement about the diet of all living creatures, especially in the light of a mountain of scientific evidence indicating that animals have been eating each other as long as there have been animals. But it is somewhat mysterious that this is one of the big differences between Genesis 1 and Genesis 9. There is possibly some sort of original symbolic context that is lost to us.
Sorry I can’t be more helpful.