Not sure about Ankoan but I can say with 100% certainty Sethrak are in SL’s. Via a major character too.
Probably means absolutely nothing aside from maybe Blizzard acknowledging how much people like the Sethrak and their lore.
But I got all the hope and passion for playable Sethrak. 
Devs specifically state that its a modified Darkspear model for both male and female PC Zandalari. I’m too tired to bring the interview up again. It was a lost codex interview though.
As I already mentioned, there’s a sethrak in Maldraxxus.
You could also make the argument that the creatures of the afterlife would have no business or interest in joining our small factions, of just one world.
Also, there’s no one saying that races have to be from current content.
They could decide to add Taunka, Ogres, or Hozen maybe, races that are already close to or inside the Horde already.
Blizzard really could add any race at any time if they so desired as they’re the ones that make the story. This is just a suggestion.
This is what I keep thinking. We’ll see but I do hope Blizzard doesn’t add them as AR’s cause it just makes no sense…
I could be proven wrong though who knows. Haven’t played through SLs yet after all.
Cool.
Still rather see Undead/worgen frame used first.
Thats fair. Sethrak use Worgen. 
Either way I’m super bloody excited about this Sethrak in SL’s. I’m dying for some more Sethrak and their lore. 