Agreed, unless they plan to recharge the "take off" batteries on the ground the total fuel onboard will be about the same as current technology. Ground recharge would likely not save much.
This is similar to diesel railroad engines that drive a generator which in turn powers electric motors to actually drive the wheels.
Although there is some loss in the conversion it has become standard rather than a direct mechanical drive as was used on steam engines.
As others have mentioned a major potential advantage of the system is being able to size the turbine for average rather than peak load. In addition to weight savings (likely negated by the batteries) this should also significantly reduce stress and prolong the time to overhaul.
The fans of course would need to cover the full range but would not be as sensitive to operation at peak load so less need for reduced performance take offs.
Of course one last consideration is the size of the fire if the batteries ever go off, would make the Dreamliner fires seem like a small spark
