Sweden’s chief Prosecutor Ingrid Isgren, has issued this statement: “This is news to us too, so we have not been able to take a position on the information that is now available. We also do not know why he is under arrest. We are following the developments.”
So, yes, he was arrested on behalf of the US and not Sweden.
Unsurprisingly, there's more than that, although I didn't expect to see confirmation so soon.