Ardern has declined to say who overheard the comment she missed, or indeed whether it was another world leader.
But she then said it happened while she was standing in a circle with other world leaders in a holding area waiting to be called onto the stage and she had missed the remark.
There is a photo of Ardern standing in one such circle on her first meeting with Trump. The other two leaders in that circle are Trudeau and Mexico's President, Enrique Pena Nieto.
The process of deduction would suggest it was Trudeau who was the recipient of a comment from Trump on the matter, and duly tittered over it with Ardern afterward.
Creating a diplomatic incident and quelling it are two different things.
Ardern got stuck in a spiral of explaining and forensic questioning about who had told her and what they said.
She was stuck between the rock and hard place - she could not dob in another world leader for narking on Trump, or ridicule Trump or admit they had been giggling away about whatever it was Trump did say. She fudged it, saying it was not enough to cause a diplomatic incident over. That is quite true.
She moved to muddy the waters, saying some reports were not correct but refusing to state which reports were not correct.
Ardern had told Sainsbury backstage during the NZ Music Awards because he was a friend of hers. She had told a longer version of the tale to at least one other person.
Ardern has since said she possibly should have just stayed quiet about it. Too right.
Ardern has learned the difference between gossiping as Prime Minister - especially when it involves a controversial figure such as Trump - and gossiping as a backbench MP, or even simply a friend.
People like to talk about what the Prime Minister told them. It makes for great dinner table conversation.
Her ministers back home were being rapped on the knuckles for over-sharing and she too had fallen into the trap.
Given Ardern had already told media her pointed retort to Trump's suggestion her election had upset people back in New Zealand ("People didn't march when I was elected"), it is a bit late to take a vow of silence now.
Ardern is no fan of Trump - she was one of those who marched against him in the Womans' Rally on the day of his inauguration.
If the reports are correct, he clearly thinks a bit more of her than she of him - The Spinoff has reported Trump reportedly said to Trudeau "you've done well for yourself."