These are time-gated mounts, but so what? Or rather, what makes this different than the numerous other time-gated mounts in game? Raid mounts like Ashes or Invincible are time-gated behind weekly raid lockouts. WoD had mounts time-gated behind garrison invasions which was also a weekly chance at rewards per toon. Legion has even more time-gated mounts if you look at the ones that come from emissary bounty caches since rep is tied behind world quests and world quests are gated by time. Venomhide Ravasaur was directly gated behind daily quests, the falcosaur mounts were gated behind daily quests, etc. Any traditional rep mount that required exalted to purchase/learn was gated behind time.
So again, why should these be different if time is the factor? They haven't announced when these will be no longer attainable if they even will ever be unobtainable. So what does it matter if someone gets them for an alt this expansion or two later?