I agree that the same issue, to different extents, applies to all non-public schools. I don't think the issue arises from being "at home" necessarily, I think it stems from the natural bias introduced by choosing activities/settings/groups to socialize in.

Home school doesn't inherently reduce "exposure" in a general sense, as you have illustrated. But it makes it easier to stick to your (or your parent's) comfort zone. If a parent is committed to not shying away from uncomfortable social situations, and enthusiastic about engaging people with different views, they can probably provide this exposure to their child. I am just saying that is definitely a challenge that should be acknowledged and accounted for. Based on many people's descriptions of home-schooling, this is a concern that is often overlooked unintentionally.