What exactly is the "arbitrary detention" that the UN's ruling on?
It can scarcely be questioning in Sweden and any possible imprisonment after due process of law. There's nothing arbitrary about than.
Nor can it be arrest and possible imprisonment in the UK as there's nothing arbitrary about that.
Nor can it be about being holed up in the embassy because he's there entirely voluntarily and could have walked out through the front door on any day since he entered the place.
I don't even see that it could be extradition to the US through legal channels as that would also require due process of law in Sweden or the UK so that too wouldn't be arbitrary.
The UK, if they don't extradite him to Sweden, or Sweden, at the end of the legal process there could deport him but presumably that would be back to his place of origin, Australia. I don't see that being repatriated is arbitrary. Would Australia send him to the US without going through a legal extradition process?
So AFAICS the only thing he can appeal about is the possibility that he could be taken to the US without going through an extradition process. How does the UN rule about something which hasn't happened and seems to exist primarily if not entirely as a possibility in his own mind?