I think they do believe it. In the end, of course, only the election itself will prove them wrong, but even then I wouldn't hold out much hope of their cottoning on to this. It was all a media conspiracy, or the voters are gullible idiots, or something like that.
Fairly sure the mentality is a repeat of what happened in the build-up to, and the aftermath of, the 1983 elections, when Labour swerved to the left and got destroyed. Perhaps the Falklands had a lot to do with this too (who doesn't love a winner?), along with the SDP-Lib Alliance taking about 25% of the vote from Labour without actually winning any seats (yet another failure of the electoral system in this country, but never mind...). But my understanding is that those on the "hard left" were determined to blame everyone but themselves for their abject failure. It may well be the same in 2020.
Or, of course, they could be right and Corbyn manages to win somehow after all. This isn't totally impossible, but I think it would have rather a lot more to do with the performance of the Tory party in the next four years than any successes of Corbyn's Labour.