This is the reason why a jet engine needs a shroud around its core: to reduce blade tip loss. When you decrease the wingtip clearance, you are curbing the wingtip vortices or the wingtip pressure bleed, and in doing this you are making each blade more and more effective at approximating its ideal i.e. infinite span high-low pressure spatial distribution under and above it, and this more ideal pressure difference accelerates the airflow better, which results in more inflow, which reduces the angle of attack of each blade a little, but increases the overall thrust produced. Think of the shroud as a big end plate for each and every blade. Think of this setup as the fan in a high bypass ratio turbofan: less fan tip clearance = more thrust.
I'm saying this only half-jokingly: I haven't read the research paper in the link, but the author better be an undergraduate writing an assignment, or he or she is surely not worth their salt so to speak, making a phenomenon mentioned in textbooks a paper.