If it was always part of Doran's plan to bring down House Lannister (and likely pave the way for Viserys/Arianne which became Quentyn/Dany) then Doran could reveal it, or Arianne or one of the Sandsnakes as they seem to have been let into his confidence now.
There's also the consideration of how Oberyn would have expected it to play out. Oberyn couldn't know someone else was going to kill Tywin. So the most likely course of events is Tywin lingers and dies painfully and people come to the conclusion it was Oberyn by poison, indeed if Cersei didn't have a hard on for Tyrion he would have been blamed for Joff's death. But Oberyn is essentially untouchable while Dorne have Myrcella, he'd be accused, probably arrested, but they couldn't execute him, not without a trial at least. He'd be able to demand a trial by combat, and of course the Lannisters would go to either Jaime or the Mountain, which evidently would suit Oberyn fine. So in his mind he wins his trial by combat and takes out another of his Elia's murderers, and despite everyone knowing exactly what he did he'd have to be let go free, which is exactly what Dorne has had to endure, knowing full well who is responsible for murdering Elia and her children but not being able to do a thing about it. It likely goes back to the snake in the grass plan that Doran talks about with the Sandsnakes and Arianne when he laments Oberyn's death.
