No, the Mech is probably mid-way to the wall by the time the rock lands, but the arm is moving before Shash even reached a position where it can hit with the rock (because Higan knows it is on the A.S.P's back and is moving to cut off its escape route) it then has to lean forward to even have a chance to damage the optics (see reference picture) which is when the A.S.P's hand is close enough to grab it. Your character outsped the arm while also delivering the blow, which suggests you've nullified my post where I said the hand would reach the Pater as its rock hit, hence the only logical explanation (disregarding all the other evident issues such as the size of the Pater and how it's even perching on the Mech) is that it could not have hit very hard, sacrificing power for speed as it were. 

I can't even make a post and move on because your chosen dodge doesn't make sense, there is really no way the Pater could fit between the arm, as far as I can guess the hole it is going through would barely be wide enough for its head, maybe, but the hole is also downward which would require the Pater to dip up over the arm and then drop down like a basket-ball. Of course, the only likely outcome from trying to squeeze through the rapidly tightening arm-space would just be a head-lock and consequent skull crushing, so it's ridiculous in every sense.

Still, as you apparently want to critically compromise your character, I'm happy to oblige and crush its head. Why it didn't take the numerous other escape avenues I will never know however.