Cheap and high no doubt, and he'll be suspended for it (as he should). The ref had an opportunity to sin bin TPJ right then and there, as they'd done last season against the Titans, but he didn't take it.
A referee reversing his decision down the track would open up a whole new can of worms, particularly as coaches will exploit opportunities to rule out an opponent's key player. And yes, it hasn't happened before and would be an entirely new rule.
Someone else who should come under scrutiny though is the Cowboys medical staff, who inexplicably let O'Neill play on for another few minutes despite copping a hit to the head and looking groggy on the ground. In fairness, most clubs will do this, but if you're serious about concussions, surely that was enough evidence to go on to warrant an immediate HIA.