According to a user report here, this is potentially still broken, even after attempting to fix it in #11

I'm not sure if there's a good way to test this, considering I already have the mount. I'm looking into getting someone else to test for me in the meantime.

Edit:
According to several reports, this item now drops from Yogg-Saron directly, and not the "hard mode" chest that was added. This means it could also drop from Timewalking difficulty (not supported yet), and also that Rarity could simply use the NPC loot method instead of relying on the now-defunct kill statistics.

Confirmed working; user must have been using an outdated version.