It was pretty awesome, but because of the camera angle, you cannot tell quite how vertical (or beyond) he really was.
In a normal wheelie, the torque provided by applying power to the back wheel provides the rotational moment about the rear axle that lifts the front of the bike. That power needs to be maintained to keep the bike up. Once the bike is vertical, any additional torque will rotate the back wheel under the front till that CoM is behind the line joining the axles, and the bike will fall. Given that the CoM of a racing bike is somewhat lower than the line between the front and rear axle, there is a bit of leeway - when the bike is exactly vertical, the CoM (and thus the rotational moment round the rear axle) should be just to the front of the bike if the rider is light enough not to provide a counter moment. This makes the vertical position recoverable. However, going beyond vertical increases the riders moment backwards, and rapidly reduces the forward moment - you won’t get much time before the backward moment takes over. At that point, to recover you need to induce a forward moment round the rear axle by braking hard enough to allow the momentum of the CoM to swing forward round the rear axle to get back to a more maintainable position.
So a power blip near vertical could start the rotation beyond vertical, followed by a rear braking action that counters and rotates the bike forward again.
Sounds pretty risky to me, probably impossible to co-ordinate, and a recipe for disaster. And you could not hold the beyond vertical position - it would be dynamic. I would need to see lots of good side-on video to believe that is is actually doable.