Hard to tell. But in this shot it doesn't look head on. I guess the refs had the best view.
If you had a freeze frame of this just a bit earlier, I think you'd see Mack's helmet on Noils. By the time of this shot, Noil is already in recoil mode.
If not targeting, it was super, super close to it. I watched it several times and I think two things likely made the call go our way. #1. mack's shoulder hit noils shoulder first and #2, noil ducked the shoulder closest to the hit which brought his facemask down and that's where the contact was.
Now, mack is still really lucky he didn't get called for it, because he did lower his head and that is part of the definition of targeting also. That said, he keeps his head up here and it's probably even more helmet to helmet contact than it already was....