That conspiracy theory is insane. No one would flee to the UK from anywhere in the world of they were worried about extradition to the US. If the US was interested in Assange, they could have got him simply by asking, when he was on police bail.
The fact is that Assange fled Sweden after he was asked to surrender to police so he could be prosecuted. He has admitted doing things which are crimes. The rest is just an attempt by his cult-followers to cast doubt where none exists.