The author thinks that the correct sex ed would reduce the teen pregnancy rates in those groups.
The author doesn't know those groups very well. They're getting pregnant because they want to get pregnant and have kids. Different sex ed won't change that.
I grew up in Texas, reddest of the red states, and a lot of my friends did abstinence pledges. Most were Christian Evangelicals to one degree or another (they even evangelized to me, though god only knows why. I wasn't really liberal at all back then).
So I know these groups pretty well.
None of them, not a single one, wanted to get pregnant and have kids at age 16. Have sex, certainly, have irresponsible unprotected sex even, but getting pregnant was not something anyone wanted. Maybe in five or ten years, but in high school, none.
I grew up with these groups too, albeit not in Texas but in the midwest and rural CA (and then some in the underbelly of Silicon Valley).
For some, having a kid got them out of the high school that they didn't want to be in anyway. (Or, junior high in some cases.) In other cases, it let them say "I'm an adult" and others thought "my kid will have to love me".
Some waited until after high school, or at least the spring of their senior year.
The author doesn't know those groups very well. They're getting pregnant because they want to get pregnant and have kids. Different sex ed won't change that.