When a thing turns habitual surely it'll kill time. One such thing had taken place with employee. It is not about gambling, as said whatever he had done apart from his assigned task during the work hours is going to stop his productivity as well as the progress of the firm. This is well understood by the Boss and he had fired the employee.
Yes, all bad habits that are done during working hours will indeed result in decrease in worker productivity, especially if it is gambling activity, they will not focus on what to do because an employee must really maintain professional attitude and of course it is their own risk when the leadership takes some harsh consequences to their worker that makes mistakes.
But in context like this, no one will really know the truth except from the leader himself and his workers, anyone can lie.
It's wrong for employees to engage in some activity during work hours but they still have their lives to live, they can do something at times during their free period during work time or break time, the nature of the job matters and so long as it doesn't affect their job performance, they are good to go in my opinion
Even this guy in the OP claimed that he didn't do it during the work hour and the court did not argue about that except for the use of cryptocurrency. But I wonder when cryptocurrency suddenly becomes an issue in a country like South Korea, if not that the judge is anti-crypto.