I don't believe citizenship is a requirement to be convicted of treason. There are a lot of members of the U.S. military who are not citizens. Surely they would be guilty of treason if they divulged secrets or aided a foreign country. People with green cards as well. Not that either of those situations apply to Assange.

I've definitely read several news articles that say he stands accused of both treason and espionage, though. I do wonder how that works for a foreigner. Has he ever even been to the United States?

Yes, none of those scenarios apply to assange. How does it matter if he has visited the US at some point in the past. It would be hilarious if he gets charged with treason. That might encourage MBS to charge Erdogan with treason!