The way I read it is that Sean Tinsley was convicted of the lesser offence of assault, by the Thai court.
Following the death of the victim, Adam Pickles, some years later, a British coroner has found sufficient grounds to allege that Mr Pickles was murdered. When a coroner's verdict is murder, the police are obligated to investigate.
As for guilt or innocence, IIRC, Sean Tinsley pleaded guilty to assault.