In fairness, Columbus thought he had found India, even after other, smarter people had told him otherwise. You can't give him too much credit, especially given that he was considered a monster even by contemporary monsters like Isabella I of Spain, his sponsor, who founded the Spanish Inquisition and still thought his treatment of the Taino natives was unconscionable. She wanted him to convert them to Christianity, and instead he exterminated them.
Those are all good arguments against praising Columbus. Especially his insistence that he had found India is a strong argument against saying he discovered America.
The specific argument “but it's not a discovery because it was already inhabited” is a particularly literal-minded child applying their teacher’s prohibitions on plagiarism to the real world.
