They very much are. I think he was just embarrassed. Also there's the possibility of it being a translation issue. There's a specific japanese word specifically used to refer to a MARRIED couple, I believe it's "fūfu" so there's a possibility that kaisei was just trying to say they weren't exactly married. There's also the possibility that because kaisei works as torano's assistant and he (umino san) is their client, maybe he didn't want him to think badly of their relationship or of torano himself.
He meant as a married couple but I've changed it to husband so hopefully it's less confusing now.
they’re not a couple?