As with Johnson, some US journalists are viewing Corbyn through American spectacles. Corbyn is not a progressive. He's a Marxist. He's not a bit like Warren and makes Bernie look soft left.
So he's not a very useful example for US election purposes. Everyone knows that a Marxist is too far left to win anything in America. No one needs Corbyn to know that. But where does electability begin on the US political spectrum? Will US voters tolerate a socialist like Bernie? I dunno. Many younger voters haven't lived through a socialist government or witnessed one overseas, so they don't know it is a rubbish system. Will voters tolerate someone like Warren who is more of a social democrat? They might have done, without her initial healthcare plan.
I suppose what I am saying is that you can't learn so much about grey areas from shades of black or white. It would be a bad idea to draw conclusions about the US election from a misunderstanding of Corbyn and Johnson.