> I assume this is why magnesium salts act as laxatives

This is a side issue, but apparently it's a different reason.

From https://ods.od.nih.gov/factsheets/Magnesium-HealthProfession... : "The diarrhea and laxative effects of magnesium salts are due to the osmotic activity of unabsorbed salts in the intestine and colon and the stimulation of gastric motility."