Do you have a source for this claim?
Canada , Sweden, Switzerland, Argentina, Spain , Australia , New Zealand and arguably undeveloped India, South Africa, Brazil, Mexico and other parts of Latin America were "intact".
Minor thoughts:
a) trivial quibble: Spain was not intact in 1940, I doubt they'd recovered that much by 1945? b) I was gonna say something bout Argentina not being a developed country, but I looked it up, and TIL that Argentina used to be super developed. TIL! c) most of those undeveloped countries had virtually no manufacturing, which is why UncleOxidant excluded them in a comment about manufacturing power
But more importantly...
The numbers I've seen bandied about (I don't have credible sources) say that the US was 50% of world GDP, and held 80% of hard currency reserves, in 1945. Supposing that's correct, even though you're technically right that the US wasn't "the only developed nation ... still intact", it's still almost-correct-in-spirit. US manufacturing capability in 1945 was way way way ahead of every other country. The point being that if the US economy experiences unusual behaviour in the period 1945 to 1971, one obvious factor is its overwhelming global supremacy in manufacturing in the immediate post-war period.
The US benefits a lot from simply being big. On some level success is simply about biomass - you need the largest number of brains you can, sitting around thinking about stuff.