Pretty fortunate. It looked a whole lot worse.
Looking at the video of his injury, it doesn't look that bad to me. It looks awkward.
But if you look at it closely, Gregory foot position means that he is putting all his downward force on the toes and inside upper edge of his foot. It appears that the foot then slips under the weight causing the shin and knee to fall to the ground. I can see how the torque of this weight goes from the edge of the foot rolling to the upper ankle.
I can also see how this might just be a mild upper ankle sprain.
The knee never buckles. Watch it in slow motion.
I can also see in real time why some might think it was an achilles as the ankle appears to collapse - but in actuality the foot slips making it look similar.
https://vine.co/v/eFtY5UKnXTW