He must have thought the Narada crew were Vulcans, right? I think it would have been cool if he had said something like: 'How can you do this? You're Vulcans!' And then Nero would respond: 'We are NOT Vulcans!', and Robau would give this confused look. It would have made clear that he didn't know it were Romulans, 'cause in keeping with canon, humans had never seen a Romulan at that point (even though I've always thought that was rather silly considering they were at war with them).