The claim here is that the morality of vegans is untenable because they believe all animals are equal to each other and all humans are equal to all animals morally. It's put in various ways, e.g. "A vegan would be unable to distinguish between killing a cat and a human child." or "Almonds kill 32000 times more animals [i.e. bees] than slaughtering a cow".
Carnists will routinely refer to humans as being somehow "in the mix" of animals, either as part of the food chain or self-comparison to lions. One is then required to ask, which is it? Are humans superior to animals or just like other animals? If we are superior to animals then such comparisons are tenuous as best because you already are constructing a completely separate moral framework, so what would it matter if we were vaguely "a part of nature"? If humans are on the same level and "a part of nature" then you can't claim special rights. The only way to square these is to recast both these arguments as "might makes right", and then you've just thrown this argument away anyway.
Almost no vegan believes that a human's life is equal to an animal's (I'm sure you'll find some strange exceptions). In fact, I think it's totally reasonable to think some human lives are more valuable than others. If you have kids, you might easily find yourself believing in a moment of choice that your child's life is more valuable than your own, or more valuable than that of an elderly, frail, or terminally ill person. The idea that all lives must be equal to believe in veganism is a straw man argument.
Almost all reasonable vegans would argue and accept that a human life is worth more than a bird's life, which is worth more than an insect's life, which is worth more than a plant's. Whether you're discussing the maximization of well-being or minimization of the suffering of "things", the only "things" you could consider are conscious beings. They are the only things that can suffer and have well-being. We also consider that an animal's capacity to experience the heights of well-being or the valleys of suffering within the moral landscape is greatest for a human, less so for mammals, less so for bees, and essentially none at all for plants because they aren't conscious.
So given the choice, it's completely reasonable to value the life of one life higher than another, and vegans do that.
It's not about whether or not the worth of an animal's life is equal to the worth of a human's life; it's about whether the worth of an animal's life is greater than the worth of your entertainment of eating meat.