It goes without saying yours is the closest analysis on the forum as to what is likely going on. He held the high ground. Negotiation goes that way when one person at the table clearly wants a deal much more than the guy and gives away his BATNA.
I think Trump, personally, did not understand that in the history of U.S.-North Korean negotiations, his meeting personally with Kim was a grand concession no U.S. president had ever before given the Kims. Yes, it was a concession.
And, inside North Korea it had huge public P.R. value in how the regime and its control is sold to the people. It marked Kim as more successful than his father and grandfather and that success was getting the U.S. to concede it was a normal country worthy of face-to-face talks with the U.S. president.
It cost Kim nothing to get that, and getting that was huge.
Now he thinks with Trump, he will still get more without conceding anything himself.