Your point with Luffy doesnt make any sense, there's no relation at all between both powers (assuming he is a Mythical DF). Luffy does that because that's his power, not the generic power of any Mythical DF. It's like saying that Marco, Sengoku and whoever else is a mythical DF can do that lol.
On the other hand, him being a Gorosei does not mean he have Mythical DF, him or any of the others. Mythical DFs are not cabbage you can find anywhere.
Lastly, we'll know if he is a "pure" swordsman or not when we saw how he fights.