As I'm reading the data... it sounds like like low vitamin D makes it harder to get enough magnesium, but high vitamin D has no effect on magnesium levels and (high or low) magnesium has no effect on vitamin D levels.

So the recommendation for vitamin D+magnesium would basically be "we think you've got low both if you've got low vitamin D, so just take both of them," relying on kidney functions to fix anything you've got too much of.