You're talking those home-schooled kids whose parents have opted out to teach them at home and they do dedicate a lot of time and energy to it. Not only do they study a greater variety of subjects without teaching the kids how to take tests but they also make sure they have a much broader perspective and practical, useful skills.
The majority of 12 million that will be illiterate fall under the category of having their parents either work constantly, have no skills themselves to teach at home, or even the equipment to do so. This means no homework being checked, no supervision of what classes they are actually present for online, no following through, no access to study material, and no activities or excursions with other home-schooled kids to libraries, museums, park, no practical life skills lessons, etc etc. Just a bunch of unsupervised at most time kids not getting any kind of education.