Agree with your first sentence but he is described as being on the very left wing of the party and, if he becomes PM, it will be with the support of the left wing, not the blairites.
By whom, and why are they worth taking seriously? Some politicians these days say all kinds of deranged nonsense about their opponents, and very often paint them as "far left" regardless of actual position. It's not worth taking at face value if it comes from there.
Obviously we shall see what he actually does if he becomes PM.
Still no specifics on who does this describing?
Wikipedia says that he is "soft left". i.e. not "hard left", the "very left wing" part of the Labour party that you claimed.