That is true, however Dialga is weak to fighting and ground, 2 types which Palkia learns moves of. I think Dialga has the type advantage here, however Palkia's stats are distributed better to counter Dialga's, with a higher special defense to counter Dialga's 150 special attack, and a higher speed stat. Overall, they are evenly useful, but in a head to head fight i think Dialga would lose, and it would go something like this: Palkia uses aura sphere/focus blast/earth power. It is super effective on dialga, who moves slower and goes for either Roar of Time or Dragon Pulse if they survive. it depends on what set the two are running. if they are both made to be fast sweepers, i think dialga loses, but if dialga is made to be bulky, dialga has the advantage. Dragon Pulse is the better idea in most cases, but here roar of time is better, doing massive damage or knocking out Palkia entirely. If palkia survives however, palkia most likely wins, especially if they get lucky with focus blast. overall, in a fight it could go either way, depending on what the trainers are using them for. base stats however, i think palkia wins, but it would be close.
