You're assuming he had a lot of choice in the matter and that the UN didn't take that into account. Nils Melzer when asked about it responded: "Mr. Assange was about as free to leave as someone sitting on a rubber boat in a shark pool." - https://twitter.com/NilsMelzer/status/1134400849097220102

Also, as far as I'm aware, Assange didn't claim he was tortured, the UN experts on torture came to that conclusion.