That is not the reason he was arrested. The assault charges have been retracted.
You may want to read this article, especially the paragraph that recounts how the Swedish prosecutors were told "Don't you dare get cold feet!!" by the British chief CPS lawyer after warning that they would have to drop the charges, and how that email exchange later was deleted, contrary to protocol:
https://www.jonathan-cook.net/blog/2018-02-12/the-uks-hidden...