I don't have anything to say on the philosophising going on above. I will, however, comment on the claim that categories should be based on in-universe definitions. I'm not sure on the policy, but we have categories like Category:Canids and Category:Equus which are clearly not based on in-universe terms, and (also, the pages in those categories are grouped based on real world phylogeny, but that's a topic for another thread). So whether we actually need a term given in-universe is not necessarily a given.