Spanking specifically means open-palm slaps on the ass, though. It's done in such a way as to inflict pain without injury (as opposed to beating).
It was literally never true that the word would refer to beating a kid by open hand only. The books that teach parents to spank also give advice to use instruments and call it spanking.
Old books that contain spanking use the word beating as synonyms. It started to be used as different word when people started to take issue with it more often.
I agree that it is not the right way to discipline a child, but I can recognize this distinction in meaning.
I find this interesting, actually, as a form of argument. The annihilation of semantic categories. It's like argument via Sapir-Whorf.