Magnesium sulfate (heptahydrate, if ya wanna be technical -- Epsom salt) and magnesium citrate (don't know its proper designation) are both laxative substances which contain magnesium.
Magnesium oxide is a common magnesium supplement. It is definitely not the same as the other two, although I don't know how laxative it is, as I never take enough of it to be "laxative", I don't think.
I don't know enough about "modern antacids", but the thing that is amusing to me is that, with Epsom Salt and magnesium citrate, you definitely will *not* feel tightness in your gall bladder area, as the muscles all nicely relax. (You might feel sore after stones have passed, but this is not at all common.) That these magnesium-containing "modern antacids" seemingly *cause* that kind of tightness is, er, strange.
Anyone else have a definite answer about this? I sure don't.