could it be that the flavor came from the sulfate and not the added Mg?
I wonder if kilning/roasting malt to various degrees alter magnesium content? Based on the article posted, it seems that an all malt wort has more than enough Magnesium. Perhaps something else in their process (at Colin's brewery) changed that altered the flavor profile of the beer?
I did a bit of playing around with my water profile for an upcoming Oktoberfest. During that time I began to doubt adding a minute amount of epsom salts would be a game changer in light of what the malt provides.
Sometimes there is so much information available that there is a tendency to miss the "big picture." Maybe this is an example?
Off topic, I'm transitioning from John Palmer's spreadsheet to Kai and Martin's spreadsheets. You guys both did a great job of creating comprehensive tools.