Hmmm, I see. I would have considered a political exemption to cover issues of political dissent, but I suspect espionage to be a gray area where it very much depends on political sensitivities. I.e. the US probably wouldn't try to extradite a UK citizen working for the UK intelligence community to not ruffle feathers, and the US simply outright won't extradite its own spies regardless. Sure, probably not fair, but there you are. Assange though is neither a US, nor a UK citizen, has definitely annoyed the US and crucially his acts if committed against the UK would count as crimes locally, which is usually one test for extradition in many treaties (haven't checked if it applies to the UK-US one).
For example, searching around, it looks like the US has asked spies to be extradited before and succeeded: https://www.nytimes.com/2018/10/10/us/politics/china-spy-esp...
I'm still not sympathetic even if he has a political defence, however. Wikileaks has published at least one classified UK document, which means he can be charged under the OSA in the UK anyway under the onwards disclosure clause (I don't know if they'll do this. Probably not, I would guess, due to lack of evidence. I do know there is a consultation ongoing to revamp the OSA partially in response to Snowden: press: https://www.bbc.com/news/uk-politics-57998950, law commission review: https://www.lawcom.gov.uk/project/protection-of-official-dat... consultation: https://assets.publishing.service.gov.uk/government/uploads/...). I think he would have stood a better chance of avoiding extradition pretty much anywhere else than the UK, including Sweden, which is why I am not sympathetic (if you're going to spit in the eye of the US, the UK is probably not one of the places to be when you do it). I'm also... perplexed, to put it charitably, as to why he'd avoid the Swedish extradition, as I think he would have had a better chance of avoiding extradition from Sweden to the US than the fight he currently has (where he has pretty much zero chance).