Going by that logic, should we consider the likes of Arcanine, Volcarona or Silvally legendaries? Should especies that can mega evolve be considered as such as well? Should friggin Wishiwashi whose BST is over 600 be considered one?
Aside from Silvally (who I already think of as a legend) none of those Pokemon are in the undiscovered egg group.
The qualities that describe all legends thus far are as follows:
1.) A limited number are obtainable in each game.
2.) They cannot breed.
3.) They have a high BST
…Except for Phione, who fits none of those. Gamefreak likes to subvert our expectations as often as possible.