I think it was a call that could have gone either way, but the reason I do not think it was called a reversal was because Chase never let go of the leg and Steven never got up on top AND behind him. Normally to be called a takedown (or in this case a reversal) you have to hook the ankle and get both hands behind your opponents arm pits. But because they were both so flexible and their bodies were so contorted that never happened. Chase had the leg held tight and was bent in half (with his nose touching his own knee). Steven was able to scoot Chases' ankle out and somehow hook it. Then he got his arm free and draped himself across Chases' back. That is why you could have called it a reversal because he had the ankle and was "on top", but he never got "behind" him, he kept trying to scoot around but never got past about half way. So they ended perpendicular to each other, but to be "behind" him you have to be more than perpendicular.
I am not sure if you can visualize what I am trying to describe, but I can see why it was not called a reversal. I thought the injustice was that the ref called a stalemate. With 12 or so seconds left, Steven had a better chance of finishing off scooting behind then he did of scoring off a fresh start.