They took him 250km away to where the hotel is and imprisoned him there. Does that sound like he was arrested for immigration troubles or this specific defamation suit?
Thai police can't speak English, so it wouldn't be out of the ordinary to ask the immigration police (which, by definition, deals with foreigners) to do it.
There is literally zero evidence that he's working here (Thailand) legally. The article only says he works here, at a school.
That immigration police arrested him rather than "regular" police, suggests that his employment may not be above board.