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:
|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/