Here is a quote in the tweet you shared ...
"The NBA is the best development system in the world, and those players shouldn't have to go somewhere else (abroad) to develop for a year. They should be in our development system."
I know he was comparing the G league to going overseas in his quote, but I think the same applies for colleges. What this ultimately comes down to is that the NBA doesn't think colleges and AAU are doing a very good job developing players and they think they can do it better themselves.
They are dipping a toe in the water now, but if they are successful they will almost definitely expand, and it would not shock me to see D1 college end up something similar to what D2 is now, and the G league becoming a better version of what D1 is now.
This is a very limited sample so take it with a grain of salt, but I watched Darius Bazely play in high school and I watched him play in the pros, and he developed a LOT in that year in between, so the NBA might be on to something.