Our benefit of slow motion replay exposes all the variants during the progression of a play.
Now, Jack that up to game speed, snap shot instant decision time.
In your honest opinion, based on the snap shot, can you say without a doubt that you would have seen the blockers left arm, then hand after the waist high block?
Improbable is the best observation. And if you had seen it all, are you sure that an ankle grab takes place? Zoomed in at slow motion, that left arm sweeps ending with an open hand that rakes along the inside edge of the sole of the defenders cleats, knocking the foot off the plant plane ultimately leading to the player going down. There might be enough to claim a grasp of the inside sole of the foot, but ankle?
The decision had been made, the flag was being grabbed and not one thought went into what that hand was doing. And if you flag for that one act...what is the flag for? Hand blocking below the waist?