Interesting to note that they set their record with battery power but are moving on to fuel cells. Discuss!
More specifically, hydrogen fuel cells.
I don't know what the energy density of hydrogen storage is these days (in kWh/kg) but if it's better than what batteries can deliver these days (400 Wh/kg for LiPo according to one of Adams posts, above) then it makes sense to use hydrogen as an intermediate energy storage solution, instead of batteries.
You do have the added weight of the fuel cell (converting the hydrogen into electricity) but I don't think they're that heavy. And hydrogen requires more infrastructure on the ground, both for production and storage, vs. an all-electric solution.
Oh, and at some point in time the general public will remember that the Hindenburg was also filled with hydrogen. That may become a significant psychological barrier to widespread adoption of hydrogen technology.