I think everyone's missing the major loophole here. I'm not sure there even is an extradition request from the US for Assange, but even if there is one, the laws an Australian citizen can be prosecuted for regarding US secrets are dubious.
The actual extradition request that actually matters, the one the UK has agreed to, is to Sweden. Assange doesn't mention that one.
As you point out, it's not offer for a some future negotiation of a treaty. And as others have pointed out, "Clemency" somehow only meaning "pardoning" isn't Assange's escape route either. But it'd be interesting if he was extradited to the US, and then promptly extradited to Sweden.