Curling into a ball is fairly natural instinct when about to get hit. Our front sides and faces are to squishy, sensitive, and crucial to survival for the natural reaction to be working something to gore us.
I believe that it would have had significant helmet to helmet contact if Cooks had not lowered his head, but obviously I can't prove it. Blaming Cooks for attempting to defend his own body (how poor a decision that turned it to be) didn't change that, I believe, it was a foul with the rules as written.
I will acknowledge that we benefited from a similar situation last week where Saffold blocked a guy in the back who had turned around at the last second. You win some, you lose some. Thompson is still an icehole.