That description is a bit tongue in cheek, but most Americans would call him far left or whatever they think is a non pejorative way to say the same thing. Just look at this very thread, you have people calling him a socialist and "truly left", they are not right wingers, or at least they wouldn't think of themselves that way, and you probably wouldn't either. And like, yeah, he has some lefty ideas, but globally speaking he runs a pretty centrist platform. I guess it's necessary in America, supporting trade unions and healthcare is as far left as you can go, maybe his personal views are a bit better and it is just a calculation.

...which are essentially the entire electorate and media apparatus supporting both parties...