i think you may be confusing magnesium with sodium or potassium
these will spontaneously ignite if dropped in water - magnesium will not ( it will burn with an exceedingly bright light if heated to white heat in the presence of oxygen - so perhaps this is where the confusion is coming from)
Whoops, I asked myself 'Why magnesium must be attached to something to be absorbed?' and that came to mind. I was confusing magnesium with sodium. Now that I think about it, wouldn't magnesium react with stomach acid to form magnesium chloride (Mg + HCl = MgCl2 + H2) which would then be absorbed after leaving the stomach. Google has the answer:
What happens if you ingest magnesium metal?
► Exposure to Magnesium may cause “metal fume fever.” This is a flu-like illness with symptoms of metallic taste in the mouth, headache, fever and chills, aches, chest tightness and cough.
many metals do that - eg welders who weld galvanised steel without suitable protection get a flue like illness from the excess zinc vapours that evaporate off as the steel is melted - zinc like magnesium is an essential mineral.
the illness is caused by too much of it in elemental form through a route we are not designed for - eg the lungs