It's not really a moral issue, it's simply the definition of the word. Using an incorrect term to describe yourself knowing it is incorrect can be perceived as virtue-signaling or claiming your own moral superiority. For example, one could ask "why call yourself a vegan if you eat fish?".

There's no policing body here, but people are rightfully calling the double-standard out here.

Edit: Would also love to hear the ethics of why fish can be eaten when cows/chicken/pigs cannot. Doesn't sound ethically consistent to me at all.