The usual direction this goes is, some cultures place higher or lower relative value on parenting your kids, and some cultures are in practice rife with absent parents as a byproduct of what they value.

edit: on rereading, I think the original comment about “natural parents” was trying to make a point about nature vs nurture, not about parenting.

Some "cultures" are just organs of a larger cultural superstructure, and they don't make sense to discuss in isolation.

I think they meant that studying children "not raised by their natural [i.e. genetically related] parents" is a way to tell that any differences are a result of parenting or culture rather than some other factor, like genetics or location.