It is extremely unfortunate but studies like this are almost always done by people with somewhat close-ended goals. This research in this one is mostly self reports, and then, mostly "did free money make you feel better?"
They are not recording, or interested in recording, the nature of who dropped out of the workforce and why, even though that seems like far more important, and somewhat more empirical, information.
In your criticism you are missing some crucial details surrounding this report.
This is essentially scrounging to make the most out of what was a single, but fully budgeted, study on BI with the intention of running more studies.
The study was cancelled by the incoming government out of spite. All people who set up plans, knowing the full length of the study beforehand were suddenly tossed out of the program and it was just down, including any study occurring.
They're trying to gather what and any information they can from the program that wasn't even given the chance to run to completion.
They are not recording, or interested in recording, the nature of who dropped out of the workforce and why, even though that seems like far more important, and somewhat more empirical, information.