He would have had to be at the men's heads to tell if Z's head was being bashed against the concrete. However, Z has facial and head injuries that he didn't have when he left for the store. There is no way Z's face could look like that if he was being pushed downwards, but the back of his head could have been bashed up with violent pushing.
MOO
In my opinion, I believe the witness testified that he could not see any actual fists or heads hitting - just movement, and the general area - so I agree, he couldn't have seen actual bashing, unless he was right there.
IMO - It's also possible that when GZ slipped on the wet grass (iirc, said so in his walkthru ), his head could have hit the walk, possibly very hard. He might have been dazed - I've done this in my own driveway, and know it can happen (yes, I'm a klutz - that's my opinion, but my family would call it fact) .
IMO - I've also had something fall out of a tree onto my head, which bled like crazy, and I would have sworn it was a tree trunk, but turned out to be basically a large twig falling off a tree branch - I was certain it was huge, because I bled so much and felt massive, but it was so small, I was embarrassed- the tree wasn't even very big or tall (this was after a storm, but I should have learned to prune the trees), but my point was - head wounds bleed a lot, even little ones, and a pretty small item can cause it.
Lastly, and IMO - I had an injury to my face that looked remarkably similar to GZ's, a little abrasion, a lot of swelling, a very bloody nose, and two black eyes, though mine teared up instantly - I was getting in my car, got distracted didn't realize I hadn't opened the door fully, and accidentally hit my face against the car door (I didn't bash myself repeatedly), but it was extremely similar to the photos of GZ - like GZ's the swelling went down relatively quickly, but the area was tender and sore, and the eyes took longer to return to normal. I'm not suggesting GZ did this, but I am saying it is possible to get a similar injury without being punched in the face.
I'm not saying this is what GZ did, but I can attest that it is possible to be injured in the same way, without having been punched. I believe there are several ways in which this could have happened, but don't believe it could have happened by a punch or slam without some dna being present on the puncher or slammers hands, cuffs, or sleeves.
They did find dna on TM's hands, but only his own. In my opinion.
IMO, MOO, JMO