You're right that there is a definition of anarchism that nobody will meet, just like there's a definition of feminism or capitalism or communism that nobody will meet. Those definitions are therefore useless, but that doesn't mean anything goes.

There's a difference between self-styled 'anarchists' who name themselves after oppressive systems and consciously include oppressive tools in their proposals for change and self-styled 'anarchists' who name themselves after systems that can help empower anarchism and that try to include as little archism in their proposals for change as possible.

The anarchist movement isn't a static definition, it's a vector force pulling at present-day society. Ancaps don't pull along that vector. Non-vegan anarchocommunists do.