But if the standard to get into school was lowered (not saying it was as I have no evidence), that it would alter the output. If not in quality then in volume.
The author's argument is that aptitude requirements did not change, but what changed is that some candidates were now less experienced after not having gone through ATC degrees. (But the author doesn't attribute the tragedy to that, in any case, so speculating about a counterfactual is something else.)