For the most part, the claim made by Richard Overy is an undoubtedly true sentiment. For instance, the greatest factor in determining which side would rise to prominence was undoubtedly economic potential and size. This is seen early in the war as being in Germany's favor as they quickly defeated the French with Blitzkrieg tactics and is still relevant throughout the entirety of the war. The Allies especially excelled in this area once the United States firmly joined the war since there ability to transform into a wartime economy enabled the transition of spending for effective military efforts, reaching a capacity of 70,000 tanks and 120,000 aircraft for use in the war. Russia was in a similar situation, defying their earlier shortcoming in military preparation, with the result of having a production system that was more abundant and of higher quality than Germany for most of the war. This economic superiority was quintessential in the Allied victory over the Axis for the strength it offered in growth as the German economy began to struggle.
In turn, the Allies also reached a superior pinnacle in how they organized and managed the military actions more effectively than the Axis. Although they did not have the same caliber of plans as Sickle Stroke for instant domination, the choices made catalyzed the potential to maintain a successful offensive/defensive for longer. The Allies had developed a capacity for setting up civilian support, constantly improving quality of objectives/technology, and back up services. In doing so, they developed a constant flow of funding and resources, the USA avidly did bonds and rationing during the era, that could be directed towards the war as the nations focused intensely on becoming victorious. For the Pacific arena the organization and depth was so strong that for each servicemen there was they had 18 people acting as support for him. The result of this, mixed with the Allied desperation to win, meant that the Allies were unparalleled in terms of strength and found a way to view the war as a "just" war that was deserving of full commitment.
With that said, the strength of the Allies would not have prevailed as easily as it did, probably even resulting in a more prolonged war, if not for the Axis weaknesses. The greatest factor that was quintessential to the loss on the German part is the deterioration of properly thought out and executed plans. The war started with quick takeovers of Poland and France, but when plans to defeat Britain were placed into action there was a loss of focus. This loss of focus in most evident in how the Nazis changed there focus from Airfields to Cities and allowed for somewhat of a recuperation period for the Royal Air Force; it was revealing of the flawed ideas and leadership since they invaded into Russia before they could truly handle it (aka a second two-front war). Likewise, the dependency of Italy on German help was a continuous drain on resources and time, the delay because Balkan assistance was imperative to the outcome to Operation Barbarossa that further weakened an economy already below par with the combined of their opponents. These weaknesses definitely helped the Allies get to their victory, but the extent to which they effected the outcome were merely in terms of the timing it ended.