That paper is pretty misleading and flies in the face of most peer-reviewed research (I don't know that journal, for what it's worth).

My guess is because it was focused on those attending elite institutions:

"In their paper on admissions to highly selective colleges... students at each of the schools in this analysis... Students opting to not submit an SAT/ACT score achieve relatively lower college GPAs when they attend an Ivy-Plus college..."

My guess is the meaning of a high or low GPA versus standardized test changes quite a bit when you have groups very highly selected based on a wealth of other information.

The Dartmouth report has always frustrated because they, along with that other paper, selectively present conditional means rather than scatterplots, hiding the variability around points to make things look more predictive than they are. Means by predictor level are almost useless without knowing the conditional variance for each predictor level. They're basically deliberately pretending that there is no error variance in the prediction equation.

Meta-analyses suggest that both standardized test performance and GPA predict later performance. For example:

https://www.tandfonline.com/doi/abs/10.1080/10627197.2015.99...

In some literature, GPA is superior, and others, testing.

There are other studies from decades ago showing that when standardized tests are temporarily removed from admissions (e.g., due to a court ruling), it has almost no influence on outcomes of admitted students later, suggesting admissions committees are able to select comparable students without tests.

I'm not saying tests are horrible and should be omitted, I just think people really overstate their predictive utility and it causes a ton of problems down the road.

Basically standardized tests (and GPAs--however corrected) are both good predictors. Depending upon the institution's objectives, other factors may play in as well though they may not correlate that well to GPA in university which may or may not be a good thing depending on your perspective. My personal opinion in that it doesn't really matter past a certain point. (You don't want people to flunk out but the objective isn't really to get good university grades.)