Earlier quoted context omitted.
You can't commit treason against another country, since you never owed them allegiance in the first place.
And yet 'Assange' and 'treason' are often mentioned in the same sentence. Which is of course ridiculous for the reason you mention, but if I were in his shoes I would be very afraid they'd find a way to make those charges (or "lesser" charges that nevertheless result into you disappearing forever) stick.
https://www.theguardian.com/world/2019/jul/25/death-penalty-...