Fair enough. Ali had assumed complete control before the supposed thumbing. Are we 100% sure there was even an accidental one? Eye trauma was mentioned even before the thumbing claims and it was extremely early in the fight too.
I think the thumb that seemed to happen in rd 4 quite likely worsened the injury to Ernie's eye to some degree. It is swelling fast in the corner after the round. But it's hard to know just what difference the possible thumbing made. Ali was after all constantly landing clean blows to the eye as well. If we compare with say LaMotta-Cerdan we know exactly when Cerdan's shoulder injury happened, why it happened and how serious it was. In short, we know he couldn't use his left after he was thrown to the canvas in the first round. One can't make out any such clearly game changing moment in Ali-Terrell imo. Not even close to it.
I don't think we can be 100%, perfectly certain of almost anything in boxing history, since we aren't dealing with mathematical proofs. The evidence strongly favors an accidental thumbing. It's better than the evidence for the vast majority of other injuries that boxers claim to have suffered in the ring. May not be as good as Cerdan-LaMotta, but it's good enough to form a firm conclusion.
Terrell said it happened in round 2. Pretty sure the commentators mentioned eye drama prior to the "incident". From memory Terrell threw his share of low blows and rabbit punches but i'd have to watch it again. "He rubbed my eye against the top rope in the second round," Terrell said. "He rubbed one eye on the rope and put his thumb in the other eye, and for the rest of the fight I was seeing two or three of him. His speed didn't bother me and he didn't hurt me with his punching, but I couldn't see him."
Yeah, I know he said that, but I don't think it's supported by the film. In rd 4 he does clearly react and complain to the ref about a thumb, though. But it doesn't seem very serious, even though you can never know these things for sure just watching from the outside.