The carnist here claims that veganism has no real definition that everyone can agree on that excludes everything it needs to and includes everything it does
Hold on, so is veganism a cult where people walk in lockstep to the beat of a single drum or is it vague and definition-less? Sounds more like carnists are tripping over themselves to come up with reasons to avoid accountability.
Name a single noun or adjective that has an undisputed definition. Take even the color red. Is scarlet or crimson "red" or are they their own colors? At what hue does red end or magenta begin? Is pure red (#FF0000) the old real red and anything else off-red? Is "light red" really red or not? "Red" is a silly example, but it's in front of me because I'm building this web page. But take literally anything anywhere at any given time. Even "chair", "table", or "computer" have definitions that not only change with time but any dispute amongst the definitions doesn't matter with regard to the ethical considerations of the action.
"Vegan" is just a label of ethics we attach to other people or products as a shortcut to whatever more specific definition you like. So who cares about the semantics of what label you put on what? The goal shouldn't be to attach a label. The goal is to reduce needless animal and human suffering.