Problem is, Trump says a lot of things that he doesn't mean. Or he forgets, it's a little unclear.
But the American intelligence community appears very interested in the guy and is known to have been working on extradition process in 2018.
If the American intelligence community wants to extradite Assange, I don't doubt somebody high-ranking in the CIA will sit down with the president and by the end of the meeting, have him thoroughly convinced that he's wanted Assange's extradition all along. They have agents trained in psychological operations and negotiation, and Trump is demonstrably an easy mark.
"UK must resist"
"Resist this attempt by the Trump administration"
and something else when he's in the van which I can't make out with people talking over it.