I think people (mainly on wizards boards) confuse Legendary creatures with creature level, I think that when mike talks about legendary creature he doesn't mean strictly a template that can be slipped on any random creature, I think it's more directed as being a category of creatures, such as Dragons or Krakens or Greater demon/devil lords or super ancient Lichs.
That not saying that you can have a Legendary goblin and I fully expect MM & Co. to give us guidelines on making those but unlike Legendary creatures they should be unique, not every goblin chief is legendary but Azzog the White, immortal slayer of Dwarven kings is.
Btw, anyone else feels like this is a freaking beast to fight? Using all 4 of its Legendary actions the dragon could fly 300ft each round and use its breath weapon each round, once the group is weakened enough it lands and start tearing into the weakest characters, and that's outside of its Lair.
Warder