Yes that's right, plants can't tell the difference between a carbon atom which came from fossil fuels, and a carbon atom which came from an animal. It's all mixed together / fungible. Nevertheless, there's an important distinction here, because emissions from food are fundamentally limited by how much food we can produce, so it's carbon balanced on an extremely short time horizon. Meanwhile fossil fuel emissions are uncoupled from any sort of sequestration, and if it balances on any time horizon at all it will be a very very very long one.