Could "energy density" not refer to energy-per-volume rather than mass? Then that headline might make sense per this quote from someone at Honda: "Simply, the energy density would be doubled. So same energy, same volume base, kind of half [the weight]"

Although the author himself confuses the matter, suggesting volume is also reduced: "If Honda's solid-state batteries truly do cut weight and size in half without reducing performance, there could be quite a bit more space in the floor of future Honda vehicles."