Your point still stands, but there's probably some small correction factor necessary since the article seems to be talking about the weight of the cells only.

Your point still stands, but there's probably some small correction factor necessary since the article seems to be talking about the weight of the cells only. Although I'd bet the weight of the cells does make up the majority of the mass of a Powerwall, other components might have significant weight. From some quick, cursory research, it seems to have a metal frame/cover and apparently has some kind of liquid cooling. (Also, minimizing weight for a Powerwall seems less important than for an EV.)