Where in the hell are you getting this idea from?
1. Federal law enforcement has no jurisdiction over state matters. If a state A.G. finds something to accuse Assange of that's a state crime, the DoJ can't do anything about it.
2. Any decision by the DoJ is reversible by the President. Except for Trump, Presidents try to give the DoJ autonomy, but that's tradition not law.
3. Any decision made by the President alone is reversible by the President or the next President.
4. International treaties ratified by the Senate are enforceable on everybody, including the states and the next President.
5. The current extradition treaty with the U.K. does not authorize anybody to waive potential criminal penalties as a condition of extradition.
Sum these five factors and you get the result I posted above.