* It's not his original argument. His original argument was that the Swedish prosecutor was biased against all men, and that sexual assault charges are a "political issue" in Sweden, so his case can't fairly be heard there.
* It doesn't make any sense legally, because Sweden's extradition treaty with the US specifically forbids extradition for espionage or political offenses, the UK treaty doesn't, and if he's rendered to Sweden for trial proceedings, both the UK and Sweden would have to authorize an extradition to the US --- but if the US decides to have him extradited from the UK, only the UK needs to agree.
Assange's case was heard before the Chief Magistrate in the UK, who rendered a detailed decision based on significant amounts of witness testimony (which uncovered an instance of Assange's lawyer overtly attempting to deceive the court by claiming Swedish prosecutors had never sought to question Assange despite documented evidence to the contrary), and again before the UK High Court (which again produced a detailed decision against him), and finally before the UK Supreme Court, which again decided against him.
I'll also note that the UN WGAD opinion contains factual errors. For instance, it claims that Assange was under "house arrest" before fleeing to the Ecuadorian embassy. He was not: as an extradition target, he was subject to monitoring and a curfew, but was otherwise allowed to leave his house at will.
It appears as if the UN WGAD opinion is really about litigating the decisions of the Swedish prosecutors, which doesn't seem like a legitimate application of the Arbitrary Detention process.