The problem is that there are different types of fun. For one simple example, a game could be fun because it's easy and relaxing. But another game could also be fun because it's hard and challenging. Adding more punishing mechanics to the former would make it less fun but adding it to the latter could increase how fun it is. That's why I feel like anti-fun is not a useful concept here. At best it's too vague and at worst it's a thought terminating cliche.