its not a DK weapon, its was never made for a DK, if it was a DK weapon then any DK could use it but that is not the case, runeblades can be wielded by anyone so why would frostmourne be any different, the raising of the four horseman has nothing to do with the weapon as there are other ways to raise the dead, blades of the fallen lore mentions nothing about having powers over the dead and the quest never even mentions the weapon at all.
and the glaives can be used by any class that can wield a sword, they only made the transmog avilable to DH as they were not around then. Frostmourne is a runeblade which can be wielded by anyone, they are not solely defined as deathknight weapons and the only person to use frostmourne is not really a deathknight he is only using the powers the sword has and becomes the lich king.
Fact is the player will never get frostmourne.