They were more sanguine about his chances before the actual primaries kicked-off, but once the campaign got rolling they really started cheerleading, which isn't a criticism since you got to keep morale up for the long fight. They might have ended up believing it whole hog and now can't acknowledge that some of it was just propaganda for the supporters.

That being said, Bernie was in a solid position to win the primary even after Super Tuesday if you just compare it to past races. But this ignores the ideological challenge he represented and assumes some kind of unbiased playing field (which has never been the case).