This is not about the fair trial. This is about someone suspected of crime in Sweden, escaping Sweden in (at least claimed and somewhat believable) fear of being extradited to US. Sweden then refusing to question him in England or at the embassy - Surely feeding that fear while keeping him stuck at the embassy. While not formally any detention it is practically so by mainly Sweden abusing the circumstances and formal procedures. This UN rule is much welcome.
That said, I have no opinion about the guilt in the underlying case. Just the abuse of procedures and circumstances.