The conclusion is, that the administrative and judicial system needs to work for the poor, too; and there needs to be a process that is somewhat practical to go from black/ gray economy to fully legal status.
He and team did historical research how that process was very similar in all countries now rich; and showed by experiment that same process is a rich privilege, or extremely costly and inefficient for everyone else (often due to hesitation to overturn nominal property rights in conflict with factual reality). The experimental study was done in a number of countries in the so called developing world, Egypt, Peru, Malaysia among them iirc, covering quite distinct cultural backgrounds, and consisted in following real life cases (over many years).