Here the carnist is engaged in minimization. "Now's not the time to soapbox about some weird diet you're on"
Why are you so up in arms about veganism? I mean if someone posted about left-handed guitars or something on a video about environmentalism, everyone would be confused about the connection. But this argument isn't confused, it's angry, as though to indicate there's some connection that people don't want to admit.
It's an ethical position, and the majority source of people's animal cruelty is their diet. You can eat plant-based and do dog fighting. That doesn't make you a vegan. So no, it's not just a dietary preference.
Think of it this way: When someone hands you a vegan menu at a non-vegan restaurant, they're saying "This is the list of things that don't gratuitously kill animals". It's not a vegan menu; it's a menu compatible with veganism.