The key part though is that the FAA was worried about the job performance of diverse candidates they brought in. They did not see a trade off between their staffing levels.
There are two separate arguments happening:
Did changing their application process create less qualified ATC controllers? Maybe! But no one seems to be arguing this.
Did changing their application process create a shortage of ATC controllers? Probably not! If anything, the evidence points to the FAA being worried they were going to get too many mediocre candidates.