To be sure. It's MANGANESE, not MAGNESIUM. And it's in a dioxide form, not elemental.
Magnesium, Manganese, ... What's the difference? Ha, ha ...I didn't know what sort of battery the ELT used, I thought it might have been something like this:
Torqeedo 24-Volt Lithium Magnesium Battery : Cabela's
A Lithium Magnesium battery. I thought I'd read, was the battery in Ancient Mariner's picture, but I could be wrong. Anyway, it didn't seem far fetched, that the ELT might use the same type of battery. Of course, the Magnesium, might not be in it's elemental state either, so could be perfectly safe.