While the rest of American commanders in the European theatre were angling for an invasion of France to relieve the Soviet Union, Churchill was the only lone voice insisting that the US Army should begin at North Africa where the opposition is relatively light (note that Roosevelt was also in favour of Sledgehammer).
Churchill felt that the green Americans should start with the less-organized Italians and Vichy French troops than the organized Nazi machinery in Europe.
No one will ever be able to know for sure so all we can do is look at the evidence of how the war went. If we look at early engagements between the US and the Germans, it is clear the US had a long way to go before fighting the Germans on equal footing. The Battle of Kasserine Pass is a clear example of green US troops struggling against more experienced opponents. The fighting in Africa and Italy helped to gain institutional experience for the US forces and helped to wear down the Germans. Another hurdle that the Allies faced early on was amphibious landings. These were very difficult to pull off and the landing of Operation Torch and Operation Husky and the lessons of the Dieppe raid were very important to the success of Overlord. The US army in 1942 was small and not all that well equipped and almost completely green and the training and fighting that the army went through before D-Day were crucial to the success of the operation.
This was really more of Sir Alan Brooke and his staff's plan, although typically Churchill takes the credit whenever he can. FM Alan Brooke was CIGS (Chief of the Imperial General Staff) and as it were, the man with the plan, and the backbone to manage Churchill's wild schemes.
According to Alanbrooke's war diaries, invading France in '43 would have been disastrous foremost because the German rail network allowing rapid troop redeployment from the east. At the time, the allies would have been stretched thin on their own shipping, because of the Italian threat in the Mediterranean. He desperately wanted to clear out the Med to free up millions in ship tonnage, and eventually he did get his way. In Alanbrooke's view, before the allies could win in France, they needed to tie down German forces elsewhere so they couldn't be redeployed to France. Further, the Allies themselves would need sufficient material and experience before they could attempt such an enormous operation.
To put it simply the allies might have a material advantage to the axis in'43, but they couldn't concentrate their resources in France faster than the Germans could. He thought landing then would be giving the Germans a chance to win by defeat-in-detail on a silver platter. I remember in his diaries he noted (with many exclamation points) that Marshall simply could not fathom this concept. I don't remember him ever really saying that US leadership was overconfident and unrealistic, but the subtext is there for sure. Even if they could put more material in than the Germans, there is the issue of leadership and experience.
American leadership (and British leadership as well)was a serious issue for Alan Brooke, and he complained repeatedly about some almost everyone he had to work with. Notably he thought very, very little of Eisenhower and Marshall's strategic ability. He understood Eisenhower to be a political general but he really railed on Marshall in his diaries because of his lack of vision.
It's hard to answer a question of "was he right" since we can't put it all in a really good simulator and see what would have happened if Marshall got his way, but I for one do think he was right. I think it was very good for the US to get real hard experience with Germans in Africa and Italy before they stormed Normandy. Charging the beaches in '43 is like asking a highschool football team to play the Superbowl. Even in '44 with all the pieces in place, Overlord was a massive risk, and there was no shortage of trepidation on the eve of it.