Well, he does lose his feet by doing exactly what you said he did. It may be thrown into the modern hip drop family but it wasn’t a classic hip drop.He doesn't lose his feet. He drops his weight to try and force the ball free and he lands on the ankle of the player with the ball.
Whatever his intent was, the end result was a hip drop. The onus is on the defender to ensure the tackle doesn't end up dangerous, he failed.