More likely just a policy that was valid 20 years ago when modern homeschooling was in its infancy, and high school graduates who’d been homeschooled were as rare as the Hope Diamond. This is nothing more than a failure to adapt.
It's probably a policy that was established years ago for holders of GEDs, which have been around for many decades, and they just put home schooling and online schools in the same category without too much thought. And now both have become more significant.