I think that it is legit because in bulbapedia, I read that the Oak's letter is already release in Japan. There are ONLY 2 types of shaymin that can get the Gracidea, one is the event shaymin, but there is another shaymin that get the Gracidea. If you use the oak's letter (in Platinum) you can catch the legit shaymin, and that shaymin can be used to get the gracidea. That means that only the shaymin from Platinum's flower paradise can activate the Gracidea. If you transfer a shaymin from D/P flower paradise you can't get the gracidea