Pottinger never said "impossible" - actual quote
“There’s a certain amount of actual dialogue that needs to take place at the working level with your counterparts to ensure that the agenda is clear in the minds of those two leaders when they sit down to actually meet and talk and negotiate, and hopefully make a deal. And June 12 is in 10 minutes, and it's going to be – you know,” the official said, according to a White House transcript. “But the President has said that he has – someday, that he looks forward to meeting with Kim.”
www.politico.com/story/2018/05/26/trump-phony-sources-white-house-briefing-background-north-korea-609617