Well, the article states that it uses magnesium hydride (MgH2), and the atomic weight of Magnesium is 12, so it means that such a compound is 7 times less weigh

Well, the article states that it uses magnesium hydride (MgH2), and the atomic weight of Magnesium is 12, so it means that such a compound is 7 times less weight efficient if we take into account the need for storing 1 Mg atom for 2 H.