It says that you have seen Dialga and Palkia because you have. Remember the cut scene at Spear Pillar at the summit of Mt. Coronet when the Cyrus uses the Pokemon of space and time to change the world to his liking? The two Pokemon are there, and therefore you have seen them without engaging in any sort of battle, congrats.
PS> I know this question was asked forever ago but I was bored and wanted the world to know the true answer.