That website smells hokum to me. The prominent ads for magnesium supplements on that page are disconcerting, as are several references to "Dr. So-and-So" with no links to anything even pretending to be peer-reviewed articles. (Actually the "About Me" page is pretty much a flashing red neon sign when you go to it).
But even discounting that, the claim that magnesium is "used up" converting Vitamin D sounds, well, wrong. The closest thing I can find to a need to convert vitamin D into an "active form" is the conversion of cholecalciferol to calcidiol and thence calcitriol. The relevant enzymes all seem to be heme cofactors using iron in the heme, not magnesium. I also don't see any peer-reviewed articles looking at magnesium impacts on vitamin D uptake. There's a few old ones (~1991, with some even earlier) that suggest that vitamin D might affect magnesium intake.
It might be that magnesium is associated with the vitamin D precursor enzymes, since magnesium is a fairly widespread inhibitor of certain enzymes (in particular, it looks like it impacts parathyroid hormone, which is also impacted by Ca²⁺ ions in the normal case of action for vitamin D). But if that's the case, it's not that magnesium is being "used up" by vitamin D but rather you need a certain amount of it in your bloodstream to keep your calcium metabolism working.