FTA:

"So it didn’t learn how to make one from the other. It learned how to subtly encode the features of one into the noise patterns of the other. The details of the aerial map are secretly written into the actual visual data of the street map: thousands of tiny changes in color that the human eye wouldn’t notice, but that the computer can easily detect."

I would not have used the word "secretly". The buildings and trees were filtered out, but not completely. The reverse transformation just amplified the noise back into buildings and trees.

Over fitting applies when an algorithm it’s self encodes the data you feed it. This is not that because it works just fine on data outside the training set by encoding the data into an output.

Further, the way it was encoding the data was steganography which is ‘Hiding a message in the title and context of a shared video or image.’ https://en.m.wikipedia.org/wiki/Steganography