A huddle which included Prime Minister Boris Johnson and the Princess Royal was filmed apparently gossiping about the U.S. president's unorthodox style on Tuesday night, although Mr Trump is not mentioned by name.
Mr Trump cancelled a press conference planned for after the summit.
New York Times reporter Maggie Haberman added that the video was sure to bother Trump because he "hates the thought of anyone laughing at him" and "for the fact that he long used "other countries are laughing at us" as an attack against his predecessors".
"In the video, Macron appears to tell an anecdote about the encounter as Britain's Princess Anne and Dutch Prime Minister Mark Rutte look on, but the French leader's back is to the camera and he is inaudible amid the hubbub".
At the start of the footage, Mr Johnson asked Mr Macron: "Is that why you were late?"
Mr Trudeau intervenes, saying: "He was late because he takes a 40-minute press conference off the top!". An amused Mr Trudeau then replies: "Oh yeah, yeah, he announced".
US President Donald Trump says the NHS is not and will not form any part of a UK-US trade deal.
Trump was asked about Trudeau's comments on Wednesday, and responded "he's two-faced".
"It's Canada. They have money". Look, I'm representing the USA and he should be paying more than he's paying, he understands it. "I can imagine he's not that happy but that's the way it is".
Earlier, Boris Johnson was forced to dismiss suggestions that he does not take Trump seriously as "complete nonsense". "I don't know where that's come from".
Trudeau in a later news conference played down the incident, saying he and Trump had enjoyed a great meeting and that the jaw-dropping comment was a reference to the announcement of the location of the next meeting of G7 leaders at the USA presidential retreat, Camp David.
"I give a lot of credit to Turkey for that, ceasefire is holding very much", he said.
Although the 29-member bloc's future is not in doubt, there are disagreements over Turkey's recent military action in northern Syria; the levels of military spending by members; and recent comments by Mr Macron that the alliance is "brain dead".
Mr Macron in turn accepted the U.S. had "over-invested" in the alliance for "decade after decade" - a long-standing gripe of the USA leader.
How sharp were the exchanges?
At a breakfast meeting with North Atlantic Treaty Organisation secretary general Jens Stoltenberg, Mr Trump said it had been a "very, very nasty" statement by the French leader.
The surprisingly gentle rebuke - at least by Trump's standards - follows concerns Trudeau's comments could result in a backlash against Canada from the president, who only this week blasted French President Emmanuel Macron.
Mr. Trump has attacked Mr. Trudeau's character before, calling him "very dishonest and weak" after last year's G-7 summit in Quebec, where Mr. Trudeau had criticized US tariffs on Canadian steel and aluminium in a press conference held after Mr. Trump had already departed. Mr Macron said he stood by his remarks.