Yes so that seems reasonable to me what do you think?
I think without seeing the CCTV footage none of us can know what went down.
I can present another reasonable version.
After the driver returned to the bus with the retrieved necklace, the thief came back to the bus to apologise. Unlikely, IMO, but that's only MO. I know that street criminals like him aren't very bright, but he'd just been chased down and physically stopped by the driver - WTF not just leg it when the driver went back to the bus?
Anyway, that was his version, but his victim was understandably alarmed, and the driver pushed him away. At that point, after being pushed, the thief made a fist, may have moved towards the driver, may actually have taken a swing, and the driver decked him.
The police take witness statements and look at the CCTV
The thief has not been seriously hurt. The CCTV footage shows that he might well have taken a swing at the driver. He is well known to them, and they take the view "about time somebody gave him a slap". They take no action against the driver.
Remember the assault did take place, he did hit the thief. It may have been lawful, or it may not have been - that was never adjudicated in court, and the police obviously decided it was lawful, or lawful enough, not to charge him, but that does not mean that he did not assault the guy.
Remember they are looking at it from the POV of did the driver beyond reasonable doubt commit a criminal offence? Apparently not.
The bus company are looking at it from the POV of did the driver follow the rules laid down in his terms of employment, or did he ignore them and end up assaulting a passenger and bringing the company into disrepute? Apparently yes.