hi frank i checked out the link and that slide talking about bioavailability of carbonate vs citrate was about *calcium* carbonate not magnesium carbonate. and there is no comparison to glycinate.
with calcium you don't have to worry so much about the runs - with magnesium you definitely do.. and hence the mag glycinate recommendation.
i was just looking at a criticism of calcium gluconate, ie not much elemental calcium in a pill.
it may also be true in the case of magnesium supplements, but it's a worthwhile price to pay if it saves you excreting it all the minute it goes in.
FYI my current mag supp delivers 150mg elemental magnesium in a capsule containing 833mg mag bisglycinate (my preferred mag glycinate was back ordered).