In Diamond and Pearl, Giratina could only be caught in turnback cave, the route to which only appears after you defeat the Elite 4 and the Champion.
In Diamond and Pearl, you only need to see every local Pokemon to obtain the national dex, you don't have to defeat Cynthia. This would contradict itself if a Pokemon in the local pokedex was only obtainable after defeating her.
And, Giratina was never part of the main game in Diamond and Pearl, Platinum was the only Sinnoh game where Giratina actually got a mention.
I hope that made sense and I hope this helped!
Source: Diamond was my first ever Pokemon game I owned, I should know, and guess who I chose as my starter!