It's not nonsense. It is a way to screen the hundreds/thousands of applicants a program gets. Just like low board scores, no honors in the clerkship associated with the specialty, poor LORs- school and degree are ways. It is similar to MGH IM interviewing everyone from Duke with a 250 step 1 score and only 1 guy from Rosalind Franklin with a 250 (despite there being 5 who applied). The strength of the school/student population and its clinical education are important when you are running a clinical residency. Just because you can memorize some facts for step 1, doesn't mean your clinical education was good enough to be a reasonable clinician.
Were the clinical education actually the same between schools, this would be a moot point. A lot of DO schools don't have their own hospital, send their students all across the country to tiny community program, have minimal in patient rotations and expect to put out a similarly respected clinical product. Were residency about sitting in a classroom and taking standardized tests all day, this wouldn't matter... but it's not.
This is about competition for programs, not about being nasty toward DOs. Those programs that don't have a lot of competition (mediocre IM, FM, psych, EM, anesthesia) consider everyone- FMGs, DOs, etc. Those programs that are very competitive, will consider only the most competitive applicants (ie those with strong board scores, strong LORs, strong grades from a school that they have experience with or is known to produce strong clinicians and researchers).
The onus is on you (not them) to set yourself up well to get a strong residency. Let's not act like people who go to DO schools didn't realize the fact that they couldn't get into the MD schools they applied to and did get into DO schools didn't mean something. In the end , from whatever school you come, all you can do is do well on step 1/clinical grades and get strong LORs and apply smartly.