Trudeau himself told her that Trump had mistaken her for his wife.
The mistake would on one level have been understandable because it was the Canadian who afforded her the introduction to many of the other leaders, more than likely including Trump.
Under normal circumstances our closest neighbour in the big league, the Australian Prime Minister Malcolm Turnbull, should have done the honours as other Aussie leaders have done with Kiwi newbies in the past.
Turnbull was no doubt fretting the absence of his kayaking cobber Key from these gatherings and abdicated his responsibilities.
So on the one level you can understand Trump's confusion, although he shouldn't have been confused.
The mistaken identity was made in what Ardern referred to as "holding pens" where the leaders are held before being paraded out for the camera for yet another mundane lineup, where they're all told again to smile - sometimes difficult considering they're wearing an uncomfortable shirt supplied by their host that'll never see the light of day again.
It's hard to understand why Ardern tried to fudge the President's confusion, fearing that she may cause a diplomatic fallout.
Trump's too busy firing insults at North Korea's Kim Jong-un to worry about such trivialities; although, given his vanity, he could have taken exception to our Prime Minister's observation that he wasn't as orange as she'd been led to believe.
John Key openly admitted that he liked to be liked and his gaffes, like 'fessing up to having peed in the shower and having had a vasectomy, added to his allure.
Essentially Key was himself, and Ardern would also do well to drop her guard and let us appreciate the woman her colleagues clearly believe her to be.