It's pretty crazy. Their whole reason for existing is to elect Democrats, and to the extent that some of the members of the group might oppose Bernie, it's because they fear he'll lose and cause down-ballot losses.
The problem here is that you can't simply disagree with people; you have to assume that everyone who doesn't profess the same views as you is some kind of nutty extremist. So (to advance it beyond the DNC, which is a fairly insignificant group here), it can't simply be "some prominent Democrats are concerned that Bernie's perceived extremism could be a liability in a general election, but I think they're mistake because X, Y, Z." It has to be "the evil Democrats hate Bernie and want to lose because of their evility. They're so evil!"