Patch Notes are up!

[Shougun]

I don't recall them saying that all Primal Mounts will fly, only that Kirin will be able to fly.

I've never had the chance to quote myself before so I had to lol, the news is recent, someone else also had the question you had :P

Unless it was accidental wording, "Ahriman and Bomb mount (from Kobolds) as well as primal horses will all fly. First one to fly will be Kirin.". Source.

Kirin will be the first, but all primal horses will at some point fly (although "first" was phrased with a question mark lol). Sleipnir would technically be a primal horse.