Also Sweden does not extradite anyone that has a chance of facing the death penalty.
And Assange said he would allow himself to be extradited to if Manning was commuted. She was, Assange is still in the Ecuadorian embassy.
The UK has much favourable extradition treaties with the US than Sweden has.
The US has over two million people in prison. Total executions last year was twenty. There is more to fear in being in a US prison than just the death penalty. The extreme sentences are also frightening.
He can claim this, allowing him to imply that he is of course innocent, safe in the knowledge that it's impossible for the Swedish prosecutor to make such a guarantee.