I have never heard of this, and in Google searching for this, I did not find anything on this.
However, it seems reasonable that in the below scenarios, in measuring the condition of a used engine engine (a) gets much worse than it's stated fuel economy engine (b) gets exactly its rated fuel economy
It would be reasonable to hypothesize that engine a is not as in good of condition as engine b.
Here is my question: How much value would you place on fuel economy as a measure of an engine's condition after a used lifespan? If a 15-year-old engine gets its rated fuel economy in the vehicle, is it likely in good condition? If it gets much worse fuel economy, is it likely in bad condition? How much information can be had from looking at fuel economy?