Moral equivalency does not imply equivalent moral understanding, and moral understanding is required to be responsible for your moral choices.

Humans are superior to animals and as such would be self-sufficient and not eat animal products.

Lions don’t have the moral agency nor the means to go vegan.

Dolphins on the other hand…

The confusion here comes from ‘morally equal’ being a vague term. Animals are equally worthy of moral consideration, but not equally capable of making moral choices and having a system of morals.

I mean, the vegans in question are also human (one presumes), so it makes sense they consider they have agency over themselves, not - for instance - over a lion which might ought to make up its own mind. And vegan humans might try and persuade other humans to embrace vegan ethics without yet making the wider step of preaching to lions (who might complain of colonialism or cultural dominance).

One presumes vegan lions, if such were to exist, would argue against the exploitation of humans for food (as in zoos), without requiring humans to conform to the same ethical standard.