Definitions can change, especially when it comes to personal beliefs of what is right and wrong. Christianity, for example, has split countless versions as people redefine what it means to them. Yet they all still call themselves Christians.
The definition of veganism includes more than just not eating animal products, but that aspect of it leaves no place to ambiguity.
What's the appeal of including vegetarians and carnists into veganism? It's not like vegans wield political power or are privileged in some way, where does this strange type of inclusivity at the detriment of reality come from?