But there is absolutely a difference, which you already mentioned: Seat in chair for 8 hrs.

They might be spending it watching YouTube but they can't volunteer or start a band or do whatever they want.

With UBI there is no nominal expectations so nobody does anti-useful things like adding paperwork to justify their existence.

The key part of the book for me is that people really truly want to work and be useful. Case in point why people accept teaching roles despite the bad pay. In fact, people will volunteer to do these things for free. So why should they get paid at all?

Jobs that are useless can't get people to stay for any reason other than money. In essence, because it is useless it pays well.

One option (UBI) is to pay everyone and then they do what they feel is most useful.

But the incentives for doing useful work are not there, since enough people are willing to be paid in good feelings instead of salary. So if not UBI something needs to change to make real useful jobs worth it