Its another garden myth arising out of old ressearch that was either mis-interpreted or misunderstood. It found completely depleted soil in third world countries could benefit from fast infusions of magnesium. There's nothing faster than adding Epsom salt.
So if your soil was deficient in magnesium, Epsom salt would be the fix. But, your soil is very unlikely to lack magnesium because it came out of a bag. Plants need magnesium to survive but in very minute amouts. If there is too much magnesium in the soil, it blocks the intake of calcium by the plant. Block the calcium and you end up with blossom end rot.