And it's not a gameplay weapon. DKs won't be running around with the actual Frostmourne at its full power.
Just as when we transmog our artifact skins, we don't run around with our artifacts at full power. Moreover, the player Death Knights were in possession of the shards of Frostmourne and we're creating our legendaries on the Forge of Origination, the same forge that created Frostmourne and the Helm of Domination.