The idea isn't that far fetched, the Patriarch is an old dragon, very old, so why wouldn't he be the same, when there's like how much, 80 or something years passing from DD to D2 ED (DKS)? I'm sure dragons lived many many many times longer, than that. And since there's no implication, that the Patriarch from DD was killed by any other means, it's safe to assume the one from Ego Draconis is the very same dragon.
