Elsewhere in his interview, McSweeney said he found Donald Trump “much funnier than I expected him to be”.
He said that during his first phone call with Sir Keir, officials in the room had been “barely able to contain themselves” with a joke the US president made about foxes eating birds killed by “windmills” – his preferred term for wind turbines.
“He went on to say that as the foxes ate so many birds and became lazy, they became fat, and as they became so fat people no longer knew what kind of a creature they were,” he added.
Asked whether Trump had been trying to be funny, he replied: “Yeah, yeah, yeah. Yeah. Definitely.”
McSweeney also said he thought Andy Burnham, Sir Keir’s expected successor, was the right person to replace the prime minister as Labour leader, adding: “I feel optimistic about it”.
He added that he also backed Burnham’s plan to create a Downing Street unit in Manchester, adding he did not think the idea was just a “gimmick”.
“If at the top of government there are people who don’t just have a desk somewhere outside London but actually live their whole lives outside of London, I think that will be a good thing,” he said.
“A lot of people won’t like it. I think it’s a good idea. I think he should just push it through, the logistics can be sorted out.”
Speaking about the end of Sir Keir’s time in office, he disclosed he had found himself too sad to watch the entirety of the prime minister’s emotional resignation speech in Downing Street last week.
Asked about his own future, he said he wanted to move in a “completely different direction” professionally, adding he had no intention of returning to British politics “in the foreseeable”.
He added: “I mean, I can’t say forever, but certainly for the next few years I’m committing to being out of politics for at least the next few years ahead.”