This is absurd. Right wing Labour had been losing vote share steadily since 1997. The previous election was the worst result in 30 years and that is not because the Tories ran a great campaign. It wasn't as bad as this time but it wasn't good.
Most of Labour's losses wasn't even because people switched to Tories, it was just because people weren't motivated to vote for Labour. Scotland was completely lost to the SNP under Labour's right wing because the SNP ran on a platform of left wing policies.
This time around could have been a walkover for Corbyn if they weren't subjected to internal sabotage from the right wing of the Labour party, as Snowden pointed out:
By the way if you look at the SNP actions it's pretty hard to argue they are left wing.
I find it hard to believe the Labour left has shifted the public zeitgeist as they didn't win in a situation that could have been a walkover for a more moderate Labour.