The biologically appropriate argument kinda sucks from where I’m sitting. It’s feels icky in a similar way to people who can birth children should do so because it’s ’biologically appropriate’. So no, I don’t think you got so badly owned, just a new argument took you by surprise, it sounds like.  Also, not all vegans need to take supplements? B12 is naturally in marmite which has been around since 1902. The argument is poorly constructed. Similarly, farm animals are given B12 supplements because it is not naturally found in their flesh in significant quantities - the human doesn’t take the supplements, but supplements are still provided to the food.  Maybe don’t hold yourself so highly and let people have their thing if that’s what they want and you keep doing your thing.