There are really two components to the EEOC letter. First one is the far more important one: "Bona Fide Occupational Qualifications" (BFOQ.) Under typical EEOC policy, a business must justify its hiring decisions by demonstrating that Requirement X, Y, or Z is, in fact, a BFOQ. In this case, the question is whether one can perform the duties of the job WITHOUT a high-school diploma.
Maybe, maybe not. It's up to the employer to demonstrate that. There are a number of manufacturing firms who utilize "pictographs" of assemblies, so their assembly-line employees can (literally) see what they should be doing at Stage One, Stage, Two, (etc.) Does that require a HS diploma for success? Maybe, maybe not.
(It certainly DOES allow those employers to hire people who are not fluent in English, by the way.)
The second issue is whether a disability which prevents gaining a HS diploma triggers ADA coverage, which then triggers the first question (above.) That's not an easy call, and it is an interesting discussion.
It is NOT just a matter of "Gummint destroying standards."
Let's put it this way: would you prefer that this individual be working in a productive job, even if low-paid, or finding his pin-money through welfare or criminal activity?