Earlier quoted context omitted.
https://en.wikipedia.org/wiki/Non-refoulement
It says it applies to the generic repatriation of refugees, not the specific repatriation of a political asylum seeker like Assange. There's also no evidence that Assange faces political persecution in the UK. His stated concern is about extradition to the USA, in which case he should apply for political asylum in the UK.
If I faced extradition to the USA, I am not sure if I would trust the UK to protect me.