I'm guessing you're right about "world's greatest athlete" being a sobriquet for the gold medalist in the decathlon, sort of how Bolt is the "world's fastest man" by winning the 100m, or how they call the winners of the Crossfit Games the "fittest man/woman on earth."
The nifty thing about the decathlon is how well-rounded, at least from a track & field standpoint, the athletes are. They're essentially doing every event in a typical Olympic meet.
The challenge with naming "greatest athlete" is that it's a.) terribly subjective, and b.) damn difficult to split hairs when talking about athletes from very different disciplines. I've seen Muhammad Ali, LeBron James and Wayne Gretzky all mentioned in this thread, and all have an argument for being the greatest of all time in their respective sports... but how in the name of The Seven do you compare those three guys, athletically or otherwise? You can't - it's a fool's errand, though excellent fodder for a friendly discussion such as this.