No where on USNA's website is the Summer Seminar listed as competitive. They have a minimum criteria for for HS Juniors for a limited number of openings for 3 seminars. Maybe competitive in the sense of first come first served with the correct criteria met. Your DS's strength of application & slate are competitive. I don't presume to know, but I do understand more. As hard as it is to accept, there's something in your DS's application/interview/essay that got him this far, but not far enough. Perhaps I'm wrong, but not going to SS did not result in a TD. An overwhelming number of appointments were made to applicants that did not go to SS for whatever reason. It seems the admissions process worked his application as long as they could.
Trying to equate two things that are not equal is a futile way on the SAF to find answers you may never find. Short of picking up a phone and asking someone who may have the answers you seek.
I understand he has an appointment to USMA, which is outstanding. Best of luck to your DS