Ardyn's royal arm if he has one, likely would be the scythe that he uses in Episode Ignis.
So I think it might have to do with a FFXIV collaboration. They found that Noctis FFXIV costume and that style of sword is pretty common for Dark Knights and Gladiators. And even though it was featured in a trailer, the textures don't look finished/ it lacks a level of detail found on other weapons. Again, considering the lack of diversity in the weapon design in the game it's weird they wouldn't include another variation, so it has to be a tie in for a later DLC or another collaboration.