That's a stretch. They base their story on exit polling. It's no surprise that those who left their homes to actually vote would skew favorably to Coons. There is absolutely no method of determining if Castle would have won in Christine's place.
Don't get me wrong; I wanted her to win, and prefer strong conservatives to run rather than the Castles of the world, but it's a bit silly to assume Castle would have lost. But I doubt a Castle win would have been satisfying at any rate. Better to have sent the message within the GOP that conservatives will be favored in the primaries.