No, this is not quite true. The malic acid helps with the absorption of the magnesium to begin with. Magnesium malate is made by reacting the malic acid with magnesium oxide/hydroxide. Magnesium oxide/hydroxide is caustic to the tissues and very poorly absorbed. By reacting the magnesium with the malic acid the magnesium is converted in to a non-caustic and better absorbed salt. Once in the body the magnesium at some point will be separated from the malic acid to be used by the body in a number of reactions.
One of the functions of magnesium is for production of adenosine triphosphate (ATP), which fuels our cells and helps them to function properly.
Malic acid also helps with fibromyalgia because it also increases ATP production.