Kanitah's strength, at that point in time, was greater than Fury's. When he slipped his fingers up underneath Fury's arm he moved his hand out of Fury's grip to slip both hands underneath. While that was a bit of an oversight, it's not one that should be noted as it's easily written off with logic. Mind you, the rear choke hold takes use of both hands, so using one hand to pin his arm behind his back would be impossible and still maintaining the same kind of hold. 

And at that point in time Kanitah was 3x Fury's physical strength, so Kanitah removing his hand from the grip isn't unrealistic. Again, however, it is an oversight.

Now, at that point in time, I didn't defend against it but you neglected to take advantage of it. You could have very easily broken my arm, and due to the oversight I wouldn't have been able to stop it. But you didn't take advantage of it. Now, with me tearing your arm out of its socket.

Fury's strength is superior, correct, but Kanitah's force is vastly superior. With his own weight, the multiplication of his own force, and the focus on your arm.

My neck hasn't been attacked because my hand has been pushing against your arm this entire time.