He was most definitely talking about Manganese, not Magnesium. Manganese originates from grain and over time goes through a redox with unsaturated fatty acids, also part of malt. Manganese isn't a yeast nutrient and isn't degraded in the brewing process, and is a powerful oxidizer. It's been studied along with malt derived Iron and Copper levels in wort as they relate to mash conversion, fermentation and flavor stability.
I wouldn't sound any alarms, though. Manganese is in every beer and the standard array of techniques to keep oxygen low and beer cold do a pretty good job of preserving freshness.
He was talking about studying hops as another source of oxidation by manganese, specifically late kettle and dry hopping, since there is no intervening boil to drive off oxygen introduced by these methods.
Just my observation, but we may have empirically figured this out, since lots of brewers are finding better hop aroma from shorter dry hop cycles.