They’re talking about level of consciousness when it’s established that the entity in question has any consciousness at all. It doesn’t mean considering those with no consciousness, like plants or rocks. (I don’t agree with it though, levels are worth considering.)
Anyone who has attended a single biology class on plants can tell that they can’t feel anything. You need a brain and pain receptors, plants lack both. But such obvious lies are perpetuated so you can keep abusing animals guilt-free.
That’s like saying morals and religion have always gone hand in hand. Can non-religious not have morals?
Plants don’t have any consciousness at all.
Being vegan is about not abusing animals. One might call it an ethical philosophy or principle. It’s not a “food philosophy”.