That's legally correct, but you're ignoring political considerations. For example, do you think that the fact that the US hasn't asked Ecuador to extradite him means that they wouldn't ask the U.K. Or Sweden to extradite him?
You don't need to have a treaty in place to extradite, a nation state can extradite as a courtesy.
If you were the US and you wanted to put Assange behind bars, why would you choose some overly complicated plan for him to be arrested on rape charges and then extradited to the US from Sweden
when Sweden avoids extraditions over political causes to the point it refused to extradite a CIA agent that defected to the Soviets? Occam's razor needs to kick in at some point.
Is there a good chance the US would like to get their hands on him? Yes.
Was he any safer in the UK than in Sweden? Nope.
What's the likelihood the rape charges were cooked up by Sweden to specifically help the US get their hands on Assange for what was obviously going to be a political reason (and which they would almost certainly block)? Very, very small.