He reveals that they aren't going back to the 4-instance heroic and normal setup of the Coliseum, instead just having plain old 10 man and 25 man normal modes. In addition, the mount will have an equal chance to drop from both encounters, so you have a chance at it no matter what your preferred raid size.
That said, you probably shouldn't expect to just waltz in and get your whole raid outfitted in matching black drakes. The mount drops one at a time, and the drop rate is pretty low. Think pre-nerf Baron Rivendare's Deathcharger mount low. As Zarhym puts it: "One day I will have it, though as soon as I loot it I'll probably be struck by lightning on a sunny day and attacked by a shark, all while in the presumed comfort of my living quarters."
That said, don't get too put out either way. It sounds like Zarhym hasn't ruled out the possibility of an old-fashioned "hard mode" achievement depending on how you face the fight (I'm fully expecting a "Many Whelps Left Side" style achievement myself), and we already know the new Onyxia is permanent, meaning you'll probably have until next expansion to try to farm your mounts.