Yeah, for things like nostr.build, something that ONLY does log-in is fine, since signing isn't needed for anything else within their service. However, NIP-46 signing can handle just logging in if that is all you need, as well.

My point is, why use proposed NIP-97 that never went anywhere and can only be used to log in, without doing anything else, when we have NIP-46 that CAN be used for just logging in, if that is all you need, but can also be used for further signing?

If the answer is that iOS won't be able to do NIP-46 signing, then that might make some sense.