Both. He does actually swallow tiny pieces of metal and burn them in his stomach. However, for our purposes, it's mostly just a fancy way to say 'Telekinesis' (or technically Metallokinesis), because I assume that he'll generally have metal-containing vials on hand the overwhelming majority of the time thanks to OM equipment-summoning.
(He can't just go around swallowing random bars of iron, though, because, well, it's really not a good idea. Burning impure metals can make him extremely ill.)
(He can't just go around swallowing random bars of iron, though, because, well, it's really not a good idea. Burning impure metals can make him extremely ill.)
There's always another secret.