Trump is just saying that the Kurds are rather irrelevant to US interests, in his view. Or perhaps that they are only relatively recent allies, so the US shouldn't feel too attached to them. He is not literally complaining that the Kurds could have but didn't help in Normandy.
Later in the same press conference Trump draws an analogy between Turkey vs Kurds with Israel vs Palestinians.
His main point repeated several times is that "Kurds are fighting for their land".
See also my own question here about locating the article that Trump mentioned as his inspiration in the press conference where he made that claim. The original article is making the point relating to WWII and other battles a bit more clearly than Trump's reproduction.
As for the other anwer (now deleted), I'm not sure the Barzani rebellion can be wholly interpreted as the Kurds fighting against the allies. Apparently some Kurdish tribes fought for the Iraqi government in that rebellion. But this is probably a question better asked on history SE. But the main point that Trumps made that the Kurds were fighting for their own land, probably applies well enough to that event as well.
Also, in Iran, after it was divided between the Soviets and the British, the Soviets encouraged Kurdish nationalism even during WWII. So it's probably more fair to say that the Anglo-Americans didn't dig that, especially the post-war establishment of the (short-lived) Republic of Mahabad, with some level of Soviet support. There are parallels to this with what the Soviets did in Chiang Kai-shek's China during and immediately after the war, even though China was also nominally a Soviet ally, e.g. their support for the East Turkestan Republic.
And since Trump's reasoning has made it to many headlines, Washington Post now has an article about it, quoting some historians chiming in (hat tip to divibisan for linking to it in the other question):
And the Kurds do not even have their own nation-state, living instead largely between Syria, Iraq, Iran, Turkey and Armenia.
“It’s a weird framing that doesn’t really make sense historically or politically,” said Djene Rhys Bajalan, an assistant professor of Middle Eastern history at Missouri State University whose research focuses on Kurds at the end of World War I. “Numerous people who didn’t have nation-states weren’t necessarily at Normandy but participated either directly in the war or in terms of providing materials and labor for the war.”
Some Kurdish fighters were among them. “They didn’t have a state, so they couldn’t act as a state,” said Jordi Tejel, a professor of history at the University of Neuchâtel in Switzerland and author of “Syria’s Kurds: History, Politics and Society.” Still, he said individual Kurdish fighters from across the region did join other armies, fighting alongside the British and the Soviet Union’s Red Army.
There were Kurds who sympathized with the Nazis, seeing them as an anti-colonial alternative to the British or French, Tejel said. But others went to great lengths to counter Nazi influence in the Middle East.
In 1941, pro-Nazi Iraqi army generals helped launch a coup d’etat, installing Arab nationalist Rashid Ali al-Gaylani as the new prime minister. And it was the Kurds who "played a disproportionately large role in [later] overthrowing that military junta,” Bajalan said.
[Quoting a tweet of Akil N. Awan]
The Kurds DID fight on the Allied side in WW2.
They helped break the siege following the 1941 pro-Nazi Coup d'état in Iraq & were part of the (pro-Allied) Iraq Levies. By 1942 Kurds made up 25% of the force. By 1943, 10 of the 44 companies comprising the Iraq Levies were Kurdish .
On that last point, Wikipedia's article on the Iraq Levies says in slightly more detail:
By 1942, the Iraq Levies consisted of a Headquarters, a Depot, Specialist Assyrian companies, 40 service companies and the 1st Parachute Company, which consisted of 75% Assyrian and 25% Kurd. The new Iraq Levies Disciplinary Code was based largely on the Indian Army Act.
By 1943 the Iraq Levies strength stood at 166 British officers controlling 44 companies; 22 Assyrian, five Mixed Assyrian/Yizidi, ten Kurdish, four Marsh Arabs, and three Baluchi.