So much in this chart was just plain wrong. I guess they checked how many people have a mount. Since the Sha was bugged for a long time and did not drop any mount it's normal that only very few people have it. But the drop rate was fixed. The mount with the lowest drop rate and hardest to get is the Big Love Rocket since you only have limited tries for a few days every year and the drop rate is 0.03%. For a 90% dropchance, you need 7674 kills. Love is in the Air lasts for 2 weeks, so that's 14 attempts pr toon pr year.
I'm thinking it's the rarest of those available. Otherwise, one of the Glad mounts would easily take that title as rarest mount.
Also I'm betting they're basing it on total accounts. While the Qiraji mount might have been obtained by more players, I can only recall only once did I ever see that mount for as long as I've played since tBC.
Yeah their list is from the obtainable mounts, it doesn't count those you cannot get any more, so it's not accurate at all=/