You're misunderstanding the argument structure. The first bit is asserting that people upset at a carnivorous animal being fed a vegan diet involves an appeal to nature fallacy because modern science allows them to receive what they need from a vegan diet. Now, I don't know shit about modern food science or feeding animals, but I can say that her assertion here is pretty sound *if* that is true. Since if the animal is fine, and the only argument is that it would typically be carnivorous, then that's an appeal to nature.
The second bit is her trying to highlight a perceived hypocrisy between people getting upset at her for feeding the animal an "unnatural" diet, while at the same time implicitly supporting industries that serve animals "unnatural" diets en masse. To simplify it, let's say that we have two things X, and Y. Let's say I love to do X and Y, and you love to do Y. You tell me that I should stop doing X because Z. I give a two part reply:
1. Z is not actually bad
2. Y also includes Z, so you are a hypocrite for doing Y but condemning X because of Z.
These arguments don't contradict each other.
Now, I don't really think she's hit the target with these points, since I imagine for most people the actual disconnect is that they don't believe that modern science permits one to feed a carnivore animal to receive what they need from a vegan diet. If they believed that, then they wouldn't necessarily resort to an appeal to nature. Additionally, I think the supposed hypocrisy she brings up next is better explained by people just believing that chickens/pigs/etc *can* live healthily on these unnatural diets. So I think I disagree with her here, but I wanted to point out that you misunderstood the argument.