It should be like other skins updated during the 20th anniversary and, depending on the update, fall everywhere.
I been wondering about this too. I assume it is probably because they accidentally tied it to Factions only. Like the Tyria specific Canthan Targe - and also in Factions - the Japan 1st Anniversary Shield. Or maybe the Ominous is just listed wrong on Guild Wiki. Either way, hope they don't change it.
The Canthan Targe is tied to Proph because it was a Proph preorder item, it's not a mistake.