The video, which has gone viral, shows British Prime Minister Boris Johnson, French President Emmanuel Macron, Trudeau and Dutch Prime Minister Mark Rutte appearing to have a laugh about Trump's behavior during the summit. But none of the leaders explicitly named Trump.
"Last night I made reference to the fact that there was an unscheduled press conference before my meeting with President Trump. I was happy to be part of it but it was certainly notable," Trudeau said during a Wednesday press conference.
Trudeau indicated that he wasn't concerned about his comments impacting the US-Canadian relationship, but Trump reacted angrily earlier Wednesday calling Trudeau "two-faced," before adding, "honestly with Trudeau, he's a nice guy."
Trump also canceled his own press conference scheduled for the end of his trip to the NATO summit. The President was caught on a hot mic of his own after the cancellation, saying, "Oh, and then you know what they'll say. 'He didn't do a press conference. He didn't do a press conference.' That was funny when I said the guy's two-faced, you know that."
"Is that why you were late?" Johnson asked.
Macron nodded, as Trudeau replied, "He was late because he takes a ... 40-minute press conference at the top."
At no time in the video do the leaders mention Trump by name.
None of them seemed to be aware that the conversation was being recorded, although they were talking openly and loudly enough to be heard by others.
"You just watched his team's jaws drop to the floor," Trudeau also appears to say at one point. Trudeau said during his press conference that the comment was made in reference to Trump's announcement during their bilateral meeting that the upcoming G7 summit will be hosted at Camp David.
"Every different leader has teams who now and then (had) jaws drop at unscheduled surprises, like that video for itself, for example," Trudeau said.
Johnson at a press conference said it was "nonsense" to suggest the video indicated he didn't take Trump seriously.
"I don't know where that's come from," he added.
Microphones could only pick up snippets of the conversation at the reception, which the press was given limited access to.
Trump also criticized Trudeau over the fact Canada does not currently meet NATO's 2% defense spending target.
"The truth is I called him out on the fact that he's not paying 2% and I guess he's not very happy about it," Trump said, adding that "he should be paying 2%" and that Canada "has money."
"I can imagine he's not that happy, but that's the way it is," Trump said.
A spokeswoman for Macron at the Elyse Palace told CNN they had "no comment. This video does not say anything special." A spokesperson for Rutte also told CNN they do not comment on closed-door sessions.
Trump spent Tuesday in meetings in London headlined by a clash with a key ally, France. He met with NATO Secretary General Jens Stoltenberg, Macron and Trudeau, making extended remarks and taking questions from the press on each occasion.
But the two leaders appeared to be on good terms as they walked onto the road leading to 10 Downing Street together for another reception following the gathering at the palace. It appeared that Trump had given Macron a lift in his motorcade vehicle commonly referred to as "the beast."
CNN's Richard Greene, Betsly Klein, Saskya Vandoorne, Kaitlan Collins and Paula Newton contributed to this report.