@JGoldman10,
One of Trump's common tactics is to attack the person when they are not in the same room as him, either via interviews or Twitter.
If you actually read the link, they mention that tactic:
"A less measured response to Trump came from Roland Paris, a University of Ottawa professor who formerly served as a top foreign policy adviser to Trudeau.
Paris wrote on Twitter: “Big tough guy once he’s back on his airplane. Can’t do it in person, and knows it, which makes him feel [weak]. So he projects these feelings onto Trudeau and then lashes out at him. You don’t need to be Freud. He’s a pathetic little man-child.”