They said previously that legendaries exclusive to an expansion should be eligible, as per the MoP cloaks.
So it makes sense given how they have confirmed the current plan is for the artifacts to go away.
The issue there is that for past legendaries it was only an option.
The artifacts are the ONLY choice you have in Legion.
How they are going to handle that for players coming into Legion content post-legion is something that should be addressed.
Keep the artifact available, ease the AP grind.
But keep baseline appearance available only to players who got it during legion, and remove access to all hidden appearances post-legion.
Resolves the legion exclusivity of the appearances while not having to improvise some other weapon solution.