@Terry
Not necessarily. Messi has been a high ranking player for a while. All he really had to do was check when there would be future World Cups to play. One was 2018, and then 2022, 2026 etc.

If he knew the status of the Argentinian team in 2015, and the various other players they had, he could have easily concluded that they weren't strong enough to win in 2018. And seeing how by 2026, Messi would have probably been too old to be at his peak, his best chance for an educated guess is 2022.

The exact date might be more surprising, but remember that these things are established many years in advance.