There is a difference, if you had been paying attention to his argument you would notice. A Legendary Pokemon is different from a pokemon that is legendary, note that the word legendary is proper in the first case, and not so in the other case. The pokemon that Neiteio mentioned are Legendaries, while the ones you mentioned that aren't in his list are legendary, because they have legends associated with them.
Legendary - Immensely powerful, extremely rare, and usually have myths or legends associated with them
legendary - Have myths or legends associated with them, that's it.
Also, I disagree that slowpoke can be classified the same way as, say, Mew because Mew is legendary across the Pokemon world, or at least across Kanto, while Slowpoke is only considered legendary in one single town in the Pokemon world.