I know it’s been that way quite a while. Maybe a dozen or so years or more?
D1 programs like to enroll their incoming freshmen into a summer school course early/mid June. Once the player enrolls into college (in summer school) the rules change regarding benefits: training, coaching, workouts, etc. and the level of contact the school can have with the athlete.
I’m not an NCAA expert, but it basically boils down to college students cannot play a game against non-college students. Which is what a D-1 enrolled player would be doing with someone who was going to a D3 school and doesn’t enroll until August.