They are indeed (although it gets a bit murky as the Realms are made up of bits of the World-that-was, resurrect souls, etc)! Sorry if I wasn't clear, I was using Bretonnia, Tilea, the Empire as examples as we don't really have much information on empires/cultures of the Age of Myth before everything came crashing down into the Age of Chaos. Part of what GW handled very badly in my opinion, they should have told us more of the glorious time before, especially in the Stormcast Eternals' case, so we'd get more impact from the devastation and a desire to see it restored...
Sylvaneth do suit Irish Mythology, Welsh Folklore, and Arthurian Legend - Tree-Revenants, Forest Folk bound by Alarielle's song and 'soul-pods', Drycha and the Outcasts/Spite-Revenants having comparisons with Seleigh and Unseleigh in Irish Faerie courts.
GW collected it as a novel and audio books that last one is the particular episode. Another thing I think GW mishandled about the launch of AoS, they seem to have decided Black Library should handle all the lore and stripped it from the game related stuff except for the barest summary...
Yes, GW seemed to have stumbled out of the gate with how they handled presenting the lore, although they seem to have worked things out, as the "Shadows over Hammerhal" game is rife with background on the life of a newly restored city of Order in the desolation of Chaos.