It looks apocryphal to me. I’ve read extensively on flea behavior, written a book chapter on it, and mentored a graduate student who did her thesis on flea behavior, and have never heard this story. It reminds me of the myth that if you put a frog in a pan of water and slowly heat it to boiling, it will boil to death and never jump out—a metaphor often used in business lectures, motivational speeches, and political rhetoric, but completely untrue. I suspect the flea story is equally untrue, but I can’t say with certainty.