Hundreds of qualified applicants were denied jobs through a "Biographical Assessment" that you can take yourself: https://kaisoapbox.com/projects/faa_biographical_assessment/ Spend a bit of time taking it and it's clear what at least one reason for this shortage is.
And yet, this utter nonsense doesn’t appear to be DEI nonsense, just incompetence. I imagine this was the result of an inappropriate and inadvertently p-hacked analysis of a silly survey of existing employees, along with a large helping of unjustified assumptions. This is the kind of thing that might be, gradually, mitigated by improved high school math education. We don’t need more calculus to fix this, and we also…
??? From the OP:
"This assessment was implemented in large part due to a push for diversity among ATCs by the National Black Coalition of Federal Aviation Employees (NBCFAE)."
And from your link:
"In 2012 and 2013, the NBCFAE continued pushing this process, with members meeting with the DOT, FAA, Congressional Black Caucus, and others to push diversity among ATCs. "
I can't see how anyone would read those two sentences and conclude "this utter nonsense doesn’t appear to be DEI nonsense". Diversity, the D in DEI was specifically the reason why it was implemented.