I'm not vegan and never will be. I don't think veganism is a crazy idea, I think it's great if that's what a person likes. And I don't see how this story, the context of it, adds substantial merit to veganism.
If cows are smart enough to be potty trained then maybe we should not confine them to feedlots and slaughter them.
If it has no friends/family who miss its absence, who cares?
Death is not immoral, it just is.
I honestly don't see how being smart enough to be potty trained has anything to do with whether something should be eaten. That's a truly bizarre connection to make.
We are part of nature and eat food that is part of a traditional diet or a very close substitute. Whether something is in our traditional diet has only a limited relationship to how intelligent it is. For instance, if it is so intelligent that we have not been able to catch it then it would certainly be excluded from our diet. But in general, that's not why foods are actually excluded in practice, so this is a fringe theory to explain why humans have developed as omnivores that eat meat as well as vegetables.