Moot point. They reported that Castle would have lost too. That’s the reality.
That's certainly possible given how badly O'Donnell lost, but I'm extremely skeptical of making that claim based on exit polling.
The reason is that I strongly suspect that the presence of O'Donnell likely increased Democrat turnout above the turnout there would have been if Castle was the candidate; the race received much more publicity because of O'Donnell; also, Democrats would be much more likely to viscerally hate O'Donnell than Castle, and thus people would have been motivated to vote just to vote AGAINST her.
So it's likely that some number of Democrats being exit-polled in DE wouldn't have even shown up to vote if Castle had been the candidate.
I hoped many were closet Republicans, but I know that's not true. Voter turnout was down, but it suppressed Republicans, not Dems.