I've never considered Phione a legendary. I've always believed that a BST of at least 600 was a requisite for being classified as a legendary. And since it also is acquired through hatching an egg, I don't consider it one.
Heh, this debate is a tough one. Personally I'd have to say yes, it is. It has a strange manner of being acqwuired compared to other legends, but the Pokedex still treats it like one. It's low BST comes not from a lack of status but from the lore making it a "younger" version of Manaphy