Did they not have an identical cohort serving as a control to compare employment rates at the end of the trial?
What they didn't do (although some wanted them to), is also check the system on already employed people and checked if that changed their likelihood of staying in a bad job/finding a better job or anything like that.
Because that's the real idea behind basic income. not just giving it to the unemployed, but to everyone, with the hope that it would encourage people to find jobs better suited to them, and to allow people to take bigger risks.