1. a group of people were assured jobs

2. the job assurance was changed to a basic income, no need to work anymore

3. people still showed up to work

errr... seems OK to me? It proves that even if there is an UBI most people will still work. The UBI is just a safety net that assures you do not die of exposure if you get to spend some time without working (for whatever reason). Your example proves that the UBI does not really distort the job market (except for the most shitty jobs, which is alright if they disappear). Also, your example proves that the existence of an UBI does not turn people lazy.

All in all, your example is a great argument for the UBI!