Theoretically the neutral for a panel containing multiple Powerwalls could be made smaller according to 220.61. However this would require a complicated calculation against the backed up loads, and I can't imagine it would ever be worth the time to do that just to save $5 on a wire size. (Especially since the calc might end up telling you that couldn't downsize most of the time anyway.) Maybe if you were installing six Powerwalls and the feeder was 100ft, but there are probably still better things to do with that time, like sell more Powerwalls.
A more interesting, and more likely in real life case, would be a 'generation panel' that contains no loads but contains energy storage and also solar inverter(s) that do not require a neutral, or that qualify under 705.95(B)*. For example if the circuit requirements for two Powerwalls and the solar inverter were 30A + 30A = 40A respectively, then the feeder hots would be rated for 100A but the neutral could be rated for 60A. And that calc takes less time than it takes to state the result.
*705.95(B) changed to 705.28(C)(2) in the 2020 NEC.