This is precisely the literary slight of hand I was talking about! Similar to Musk's hilarious dismissal of hydrogen technology. Hydrogen will obviously in time become the ideal source of energy for electric aircraft and electric long haul trucks.
Used a lot in mathematics, substituting "proof" for "evidence".
Efficiency depends on fuel cell type and operating conditions and such, but I think it’s generally pretty high for the useful life of the catalyst material. (The catalyst is another materials challenge itself, since I think most of the good materials are super expensive like Pt/Pd alloys, and they’re vulnerable to carbon monoxide poisoning)
Edit: this DOE program page has some good detailed discussion of the current status, including on compressed liquid/gas storage systems, which I’m not as familiar with: https://www.energy.gov/eere/fuelcells/hydrogen-storage-curre...