Yeah, that is what they say. He is not a U.S. citizen. The conditions he is being held under have been observed by multiple witnesses, and they are appalled. He won’t survive, which is what the global powers want.
What part of what is happening to Assange is happening are you struggling to believe? That he is subject to extradition proceedings in the US and that he skipped bail for years holed up in the Venuzualan embassy are demonstrable facts, and I notice Trump hasn’t done anything to halt these proceedings. As for why he is in protective custody, you can imagine why they might want to ensure his safety considering the conspiracy theories that would surface if he was murdered.