The US has not made any charges against him, and they have not requested his extradition from any country. It would be silly for Sweden to promise this. If anything, it would only encourage the US to actually test such a promise, even if they had no intention to do so.
Secondly, Sweden is bound by the terms of any treaty it signed with the US. They don't get to cherry pick who they will decide to extradite and who they will not. It is not even a political decision - a court will decide whether to extradite him or not based on the agreed rules/laws on extradition. If Assange was in Sweden, and he met the criteria for extradition, then he would be extradited.
And what exactly is Assange bargaining with here? Usually, a deal to avoid prosecution is made when you have something to bargain with. If Assange wants to bargain, he needs to offer something of value to Sweden. Now if he has some yet-to-be-leaked documents, Sweden might be willing to make a deal.
The man is simply using the extradition as a smokescreen. He is right up there with Polanski in my opinion - a fugitive from justice.