No, I am aware of that fact. It's just that if no one had access to Manaphy, who I believe is only obtainable through the Ranger series, you would be unable to access Phione. SO to compensate for that, the developers made Phione unnecessary for completing the National Dex.
Your original question was " is it needed to complete the National Pokédex ", which I am saying no because of the requirement that you get Manaphy or have access to someone who does. Wonder Traded Phiones would not be possible if no one had a Manaphy, right?
It is possible to Wonder Trade or trade for Phione, all you have to do is to go to GTS, then choose the Pokémon you want, then you type the name of any Pokémon that was not discovered yet and then you can have Phione. Also, it's impossible to trade event Pokémon and there is a medal for completing the National Pokédex which excludes event Pokémon except for Phione, so you don't need a Manaphy, so you just trade or Wonder Trade it.