Okay that's a good point. Also I love the DF reference, DF is my all-time favorite game.
My original comment somewhat lacked empathy, perhaps because I don't suffer from problems with free time fragmentation because I'm always possessed by a Strange Mood. I still think my point stands though. I meet a lot of people with what I know to be fairly large chunks of time that don't do anything with them anyway, not because they feel like they can't achieve something in that time but because they don't have any hobbies or ideas, or particular skills. In fact, that's true for most people I meet, and it's actually difficult for me to tell whether that's an artifact of perceived free time fragmentation or not. And this is in a culture where people are 100% encouraged to develop skills and hobbies.
But think about it, as much as we hear about retired people doing epic things, we also don't hear about retired people, like my grandparents, being terminally bored and ending up back to the workforce.
Here's the thing. As much as jobs can be boring and soul-sucking, to an extent they provide more of something then doing nothing while on UBI does. In addition, they provide a lot of training for life, like perseverance through boring things, and people skills (this is my experience at least) that you can't really get anywhere else. Plus, while jobs might be unfun, developing any kind of skill and competency at something genuinely gives people a sense of self-worth. Often, jobs provide the sort of skill that is easier to master than, say, art or poetry. Unless UBI comes with some sort of education or structured skill development, it really won't be good for people in the way you're suggesting.
I'm willing to consider it possible however that there might be more people capable of learning skills and achieving self-actualization than we currently see manifest in society, and that with UBI those people will be freed up to do things. I just don't think that it's a totally positive thing. Besides all kinds of other logistical issues and issues like it ending up subsidizing drug addictions and such.