Earlier quoted context omitted.
The problem is as never the tests. It was pretending that the difference between a 600 and 625 (or whatever) really predicted anything. It was the silly idea that with tests you could produce a fair ordering of students based on potential to succeed.
You can absolutely make a bet on who's more likely to succeed based on a 100 point difference, though. It's not absolute, but it's highly predictive. And the reason the SAT was dropped wasn't because admissions were being forced to blindly accept 620 over 610 (they never were), but so that people who scored hundreds of points below the mean could be admitted (in the pursuit of other institutional goals).
Flip answer: the bucket width should be 2.5 times the score improved of a prep course.