I believe this story was popularized in the 1965 film Battle of the Bulge, in which a German officer (Colonel Hessler, played by Robert Shaw) finds a chocolate cake in the possession of an American POW and concludes that if the US can take the fuel and time to ship something as trivial as a cake into a war zone that Germany will surely lose the war.

I don't know if the incident really happened, but I believe the above movie popularized the story of it.

Not this situation specifically, but there are always stories about the "enemy" being discouraged upon finding a trivial item.

One that I know specifically, is a story from the book Nothing to Envy: Ordinary Lives in North Korea. This anecdote takes place during famine and suffering in the 1990's.

A North Korean soldier would later recall a buddy who had been given an American-made nail clipper and was showing it off to his friends. The soldier clipped a few nails, admired the sharp clean edges, and marveled at the mechanics of this simple item. Then he realized with a sinking hear: If North Korea couldn't make such a fine nail clipper, how could it compete with American weapons?

Variations on this story pop-up on the internet, with the solider finding the nail clippers, and being sure that someone was looking for such a valuable item. When no one comes, he realizes the nail clippers must be extremely common, and loses hope.

Sorry, forgot to add: The remark implies the massive logistical capability of the US that it can send a perishable and non essential item thousands of miles to the front, which Germany cannot match.