Graeber actually points out that centrally-planned economies had formally higher work hours than was the reality (people slacked off more), and that it was akin to workers collectively making a choice working less for the overall lower production.

But the obvious problem is that it's still a societal taboo, so it's not UBI because it's not universal. It's probably lot more efficient to actually openly embrace shorter work hours, so that people can stop pretending and choose to do something more useful.

Also, existence of BS jobs is often driven by high status individuals, so it is comparatively less free than UBI.