The MC's consent was violated. Because he believed he was fucking his wife, not the twin sister.... so again, violation of consent... therefore... rape tag.
If we were a legal advice website, I'd agree with your logic. But nobody who is into rape porn is going to be jerking it to this.... "YEAH BABY, YOU MAKE THAT MAN FEEL A VAGUE LACK OF AGENCY! YOU DIRTY BITCH!" And on the flip side, nobody who is adverse to rape porn is going to run screaming from their screen over this.
The purpose of tags is to help people find (or avoid) certain kinds of pornography-- not help people work out legal definitions in whatever country the tagger happens to live in.