I think the author is missing an important point: like many young people, he was smart but undisciplined. If he had gone directly to college from high school, instead of into the military, he probably would have performed as badly in college as he did in high school. He needed more time to grow up and mature. The SAT is irrelevant to that natural fact.

If you're not self-disciplined, then college can be even worse than high school, because you have more personal freedom in college. For example, if you skip classes in high school, you're in big trouble, but if you skip classes in college, they just shrug, cash your tuition check, and give you an F. In college, the adult supervision is largely absent. You're expected to be an adult and supervise yourself.

The military accepts smart underachievers because the military is going to force you to work whether you like it or not.

I didn't join the military myself, but it took me until age 24 to become self-disciplined. Although I was smart enough to get into college, I wasn't psychologically ready for it at 18, and I dropped out at age 20. I had a great time partying for the first 2 years though. ;-)