MLS/International Soccer Academy Model
I think what we are seeing is the evolution of the NBA towards the Academy model of international soccer and now the MLS. The teams (or in this case the league) sponsor training squads where the players are paid (see Tyrese Proctor as a basketball corollary), and some of the most talented go pro early and others go to college for additional training as they age out of the academy. I haven't checked, but my guess is that a bunch of Duke's soccer players were in MLS or international academies, and either need more maturity, or just aren't ready for the pros yet. But some do well (Thor for Duke last year) and may go pro quickly, while others stay the full four years. What the NBA is doing is getting more control over the players, and prototyping what I think will become an academy model to compete with NYBL or AAU teams.