Not going to dispute the original source, but I'll just say that this is nothing new. The "Greatest Generation" included a lot of really dumb people. Not bad folks, just very ignorant, as one might expect for people who grow up sharecroppers in Alabama or in some tenement house.
Even listening to the Easy Company veterans in Band of Brothers, you can tell they're not exactly endowed with high educational prowess. That being said, they performed extraordinarily well. That's because courage has little to do with intelligence (it may indeed have an inverse relationship more often than a correlation).
I doubt very seriously that we "lost Vietnam" because of the low IQ of our troops. Low IQ will show up on the battlefield in a few instances, and that might be part of the cause of things like the Mai Lai incident. It also becomes a bigger problem the more technologically advanced your military becomes. But there are so many diverse reasons for our failure in Vietnam that this sounds like a huge dodge, especially coming from someone who made key decisions.
It sounds like some of the cases here are extreme ones, like people with Down's Syndrome or something being in combat. I'm sure that's bad, but that's a relatively small piece of the iceberg. Flat out ignorant is much more common, and it STILL is. If anyone tells you that we're better now because we require a high school diploma, they're full of crap because a high school diploma now means nothing. A 9th grader in 1960 is probably smarter than a 12th grade high school graduate today. And ultimately, performance on the battlefield is about courage and doing the tasks assigned, few of which are tasks where a high school degree is relevant.
I'll just say as someone with the most recent on-the-ground battlefield experience (1.5 months ago), that there are quite a few morons in the military and they somehow manage to not die on a regular basis.