The Hydrogen Economy is a completely different animal to the Magnesium Economy. Both hydrogen and magnesium can be used to transport and store energy but the advantages and disadvantages are very different, thus:

	Hydrogen	Magnesium
Cost of production	Higher	lower
Storage	difficult and expensive	easy
Efficiency in use	best fuel cell 60% Needs expensive catalyst	better than 90% Cheap
Transport	Can be through pipe	Shipping or other systems
Economic use	Just another commodity	Good for global bartering
Weight as transport fuel	Dominated by weight of storage system.	Weight of magnesium and cell
Reaction products	Non polluting water. Can be discharged safely; zero weight at end of journey	Are retained in cell which end up heavier at the end of journey

Hydrogen would be used directly in the manufacture of steel and cement both of which are currently hard to electrify, so its role should be seen as complimentary to magnesium rather than as a rival.

If a sufficiently low weight storage technology can be developed, it would find application in air transport.

The overall cost issues for Hydrogen are complicated, See https://insideevs.com/news/332584/efficiency-compared-battery-electric-73-hydrogen-22-ice-13/