No one witnessed him saying that. Even John Bolton who isn't a friend of Trump said it was false.
All the Atlantic magazine had was unknown sources or hearsay.
John Kelly confirmed it to CNN - this is from NBC. Did you miss this?
"A person that thinks those who defend their country in uniform, or are shot down or seriously wounded in combat, or spend years being tortured as POWs are all ‘suckers’ because ‘there is nothing in it for them,'" Kelly said of Trump. "A person that did not want to be seen in the presence of military amputees because ‘it doesn’t look good for me.’ A person who demonstrated open contempt for a Gold Star family — for all Gold Star families — on TV during the 2016 campaign, and rants that our most precious heroes who gave their lives in America’s defense are ‘losers’ and wouldn’t visit their graves in France.”
Trump called fallen soldiers "suckers” and "losers" during his presidency, according to a 2020 report in The Atlantic.