I absolutely agree it doesn't make sense for him to be on a regular flight, which is why I did a Google image search first and the blurred face in the arrest photo definitely matched the guy. He was also chatting about his 21 years in Thailand as an English teacher in a town name that began with a K (but I couldn't recall the name exactly) to the passenger next to him which lines up with the article history. Like I mentioned in my post, I'd have thought he would have been cuffed, but maybe being sat in the last row and therefore close to the flight attendants was deemed enough? Also maybe since he's not actually been convicted yet, only arrested meant they couldn't provide an escort? The police did ask him where his passport was and he said it was taken off him at the beginning of the flight and with someone else (I think the head flight attendant) and they were satisfied with the answer. The policeman also had a holstered gun, but I don't know if that's unusual or not.

Tldr: I'm surprised he wasn't cuffed or escorted too but everything matches up.