If you call WWII doing nothing then I guess that theory is correct.
I think you missed the timeline.
???Before I read that, I had never known of a theory connecting the Depression of 1920 (back then, a “depression” was the normal label for an economic contraction. They thought it sounded more reassuring than “panic,” which was the name used before they had a really bad panic in the 1890s) to WWII. My understanding of the history is that rumor that the Smoot-Hawley Tariff - a tax increase - would be enacted and signed caused the crash of 1929. Hoover signed it over the objections of economists who warned that it could lead to “a second world war.” Then, when the economy was in the tank, Hoover started the New Deal. He didn’t call it that, of course - that was FDR’s formulation - but if FDR had been president in 1929 he wouldn’t have been able to ramp up spending any faster than Hoover in fact did. And after running to Hoover’s right during the campaign of 1932, FDR proceeded to double down on everything that Hoover was already doing wrong - leading to the extension of the “depression” into the Great Depression. Which continued until FDR was “scared straight” by Hitler.
The US economy began recovering from the Depression when Britain started buying any war materiel which we could sell them - a process which dramatically accelerated upon the Fall of France in May, 1940. Having been under secretary of the Navy during WWI, FDR knew that America’s production capabilities had made no difference to that conflict because the US entered the war from a standing start, and was only in the war for about a year and a half.
FDR knew that the US couldn’t afford to enter WWII from a peacetime economic basis again, so he asked Bernard Baruch to be his mobilization czar (without using that title, but that is the sort of extra-constitutional position he had in mind). Baruch demurred on grounds that he was too old. FDR asked who he recommended for the job, and Baruch replied, “two names. Bill Knudsen, and Bill Knudsen.” (A Norwegian immigrant, Knudsen had become Henry Ford’s right hand man before Ford’s other right-hand-man forced him out. Knudsen then joined a struggling auto manufacturer called “General Motors” - and made Chevrolet the equal of Ford).
FDR asked Knudsen, a production genius, how long it would take for America to mobilize for WWII, and Knudsen said, “Eighteen months.” By the merest of coincidences (if you believe in that sort of thing), that is almost exactly the time from when Knudsen spoke until December 7, 1941. After the attack, FDR asked Knudsen how many planes America could build in 1942, and Knudsen replied, almost 50,000. FDR promptly went on the radio and pledged that America would build 50,000 planes in the coming year. But of course, America was caught with very little armament inventory in December, 1941. That was for the simple reason that while it had ramped up production over those 18 months, America had dedicated that output to keeping Britain afloat. BTW, the technology dump Britain sent to America - radar, the jet engine, the Merlin engine blueprints, lots of stuff - happened upon the Fall of France. But, even something you would think was highly specific and “shovel ready,” the Merlin engine design, had to be completely translated from British to US production technology (by Packard).
All of which is to say that WWII disrupted the New Deal, putting productive output on a much higher priority than “social justice,” and that is what ended the Great Depression. FDR himself said that he was changing from “Dr. New Deal” to “Dr. Win The War.” The New Deal wasn’t the cure for the Depression, it was the cause of the extent of it. Think of the choice you were faced with if you were a voter in 1932! Either the guy who caused the Depression, or the guy who would keep it going for the rest of the decade!!