Different sources, different reasons of arrest. But whatever it is, it has something to do with money for sure. If it'z embezzlement of funds, then he obviously wants to follow the footsteps of SBF, but unfortunately not in volumes and amount that SBF did it. If it's due to price manipulation, there's no way the law enforcement is pinning him solely for this activity. I don't think there'd be a lot of panic with the CEO's arrest. Bithumb is one of those exchanges that receives the customers that do not want to deal with the large ones, so its market share is really not that big at all.
i have used that exchange before but just like what you said, they do not have the big share in the crypto market industry. however, if you are a user of this exchange, better get out your funds while you can. you don't know what will happen next. even if it is only the CEO's fault about money matters. they can easily freeze your funds without warning.