Also a possible construct. The key point here is that nothing about the known background of Frostmourne changes - it was given to the Lich King by the demons, and supposedly the Nathrezim forged it (a point that's briefly mentioned by never explored), but instead turns out they stole it or else it's true origin lies elsewhere (in the Maw). If there was some detailed story of a Nathrezim smith or Kil'jaeden himself toiling over the creation of the blade then I'd agree it's a significant retcon. Without that, though; I file it under expanded history as opposed to retired/retroactive continuity.