This was done before the world cup draw too. There was some suspicion that the tweets accounted for all possible variations and were made private at the time of writing. The account could simply have deleted the wrong 'predictions' and them made the account public after the draw thereby appearing to have correctly predicted all of the matches before the draw was made.
Probably aren't all that many combinations anyway. Especially if so include man city v schalke and schalke v man city as one fixture (note that they have home and away teams wrong). You can't face the other team from your group, you can't face a team from the same country, limiting how many each team can face, and I imagine there are probably only a combinations available once that's taken into consideration.