Its just an ad-hoc claim invented to keep their dogma.
According to
Genesis 4:4:
"And Abel also brought an offering—fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering..."
He did not bring wool, but fatty meat (best parts for eating).
a. You have not addressed what the text said about initially giving plants to eat, and now giving them moving things. What do you think it was supposed to mean?
Genesis 1:29 Then God said, “Behold, I have given you every plant yielding seed that is on the surface of all the earth, and every tree which has fruit yielding seed; it shall be food for you; 30 and to every animal of the earth and to every bird of the sky and to everything that moves on the earth which has life, I have given every green plant for food”; and it was so.
Genesis 9:3 Every moving thing that is alive shall be food for you; I have given everything to you, as I gave the green plant.
b. Regarding Abel, yes he was already tending the flock.
Genesis 4:2 And again, she gave birth to his brother Abel. Now Abel was a keeper of flocks, but Cain was a cultivator of the ground. 3 So it came about in the course of time that Cain brought an offering to the Lord from the fruit of the ground. 4 Abel, on his part also brought an offering, from the firstborn of his flock and from their fat portions. And the Lord had regard for Abel and his offering
You present no evidence of people eating animals in the text prior to the flood. Sacrifice was the use made of the animal in the passage. And if they were regularly sacrificing, then having a heard to do so would make sense.
Whether they used the flock for wool, etc. is not mentioned. But we do know that they were provided garments of skin by God in the garden. So the notion of using animals for practical uses other than food is not that unusual.
Genesis 3:21 And the Lord God made garments of skin for Adam and his wife, and clothed them
We only have limited information about the period immediately after the flood. But there was already a clean and unclean status to animals prior to the flood, referenced in the text. Since after the flood the animals are said to be given as food, then the likely explanation is that prior the distinction of clean and unclean dealt with sacrificial animals.
Genesis 7:2 You shall take with you seven pairs of every clean animal, a male and his female; and two of the animals that are not clean, a male and his female; 3 also of the birds of the sky, seven pairs, male and female, to keep their offspring alive on the face of all the earth.
Sacrifice was again a use made of animals when Noah got out of the ark, prior to the giving of animals for food. And it is clearly associated with the clean animals.
Genesis 8:20 Then Noah built an altar to the Lord, and took some of every kind of clean animal and some of every clean bird and offered burnt offerings on the altar.
c. You reference mythology, so I am not sure how you view the meaning of the text overall. But in any case, elements of the text of Genesis indicate that the book is written from the perspective of someone familiar with at least some of Israel's founding and history. So descriptions of fat portions would not be surprising, from the perspective of people who did eat meat, but also were familiar with sacrificial offering, and its language, which uses similar terms.
In any case, the use of animals as sacrifices and clothing are both concretely referenced in the text, while use as food is not. And a statement is present that indicates God gave plants for food. And another, later, statement is given that states animals were given after the flood, as plants were before. So it will take more than just saying that the presence of herds, and reference to fat portions, demands that people were eating animals at the time. This is especially true since the account is written for a people that lived in a time when animals were food, (and understood what fatty portions would entail for food, and sacrifice), but seem to reference a time when meat was not food. They would have little reason to downplay meat eating in the pre-flood times.
