If I Google how many adults in the US identify as Christian the answer is that in 2019 65% of those polled identified as Christian of fractional under 2/3. It seems reasonable then the 2/3 of homeschooling parents would identify that way. While the parents may state religion as one reason for homeschooling it does not follow that religion is the primary or deciding reason.
People who just identify as Christian (but who may not go to church much, or may attend a moderate church) are not the same as the extreme fundamentalists that are associated with homeschooling in the US. (When people found out that I'd been homeschooled when I was a young adult, they tended to assume I had this background.) If you haven't met these people, or heard the testimony of a survivor of this kind of homeschooling, you don't understand this issue.
I am on forums in the US and in a Christian homeschool group in the NZ and acquainted with a lot of non Christian homeschoolers. I haven't heard one give religion as the main motivation. I am sure in some places their are a lot of fundamentalist Christians homeschooling for reasons of religious conral but whatever Harvard claims it is not 90% and it is not 2/3 either.
The population of people on homeschooling forums is not the same as the population of homeschooling parents. I don't think a lot of the kinds of people I'm talking about would be found (at least in more mixed spaces) on the internet.
Growing up, I did not know a single homeschooling family other than my own that was not homeschooling because they were extreme fundamentalist Christians that wanted to shelter their children from secular influences. They used only fundamentalist textbooks in which "science" was mostly about teaching creationist debate points, "history" was about glorifying historical figures that advanced their ideology and vilifying historical figures that didn't, and "health" was about teaching falsehoods about sex and abortion.
The kids I knew growing up weren't even the most sheltered. I had a coworker at a camp when I was in college who was a homeschooled 17-year-old, and what shocked me about him was not just his appalling ignorance about everything but the Bible, but that he'd never really interacted with a woman/girl other than his own mother. I don't think he'd ever used the internet.
