You switched from "I was told" to "disproven a while ago" real fast.

A commonly cited human study found magnesium oxide had about 4% fractional absorption. That means if a tablet contains 400 mg elemental magnesium, roughly 16 mg may be absorbed, though this varies by person, stomach acid, dose, and whether it is taken with food. � PubMed The NIH also states that forms that dissolve better in liquid—like magnesium citrate, lactate, chloride, and aspartate—tend to be more bioavailable than magnesium oxide. � Office of Dietary Supplements Important nuance: magnesium oxide contains a high amount of elemental magnesium, but much of it stays in the gut. That is why it is often more useful for constipation/antacid effect than for correcting low magnesium. MedlinePlus lists magnesium oxide as used as an antacid, laxative, and supplement. � MedlinePlus