What the ref says is conjecture on my part based on the coaches reaction. I assume they say he wasn't defenseless because he rolls his eyes and walks away.

But their decision makes no sense if he was defenseless. Here is the rule:

"No player shall target and make forcible contact to the head or neck area of a defenseless opponent with the helmet, forearm, hand, fist, elbow or shoulder. This foul requires that there be at least one indicator of targeting. When in question, it is a foul (Rules 2-27-14 and 9-6). (A.R. 9-1-4-I-VI)"

So the elements of a targeting call here could be broken up as follows:
It's clearly forcible contact to the head: check
A defenseless player: check (you are assuming this is what they decided)
With the helmet: check (it was definitely helmet to helmet)
At least one indicator of targeting exists. So this is really the only question here.

The rule provides a non-exhaustive list for targeting.

"Some indicators of targeting include but are not limited to:

--[Launching]. . .

--A crouch followed by an upward and forward thrust to attack with forcible contact at the head or neck area, even though one or both feet are still on the ground

--Leading with helmet, shoulder, forearm, fist, hand or elbow to attack with forcible contact at the head or neck area

[Crown of the helmet] . . . ."

I think the second one is likely present, but the third one is obviously present. He lowered his helmet as he went into the tackle, first contact was the helmet to helmet and it was certainly forcible contact. I don't know how you don't call that target with if he was a defenseless player.

Oh and if you aren't sure whether you agree with me or not, don't forget the last sentence of the rule: "When in question, it is a foul."