Presumably it is a balanced source of power. I know my Enphase micros do not even have a neutral. Do we even know if the Powerwall has a neutral? I know my hybrid inverter has a neutral because it is connected to the subpanel which may present unbalanced loads.
Powerwalls definitely have a neutral connection. However, I don't know for sure whether they balance the neutral through an internal transformer or by electronic means in the inverter(s). Without much evidence either way, I assume there is no internal transformer.
In the disassembled photos that I have seen, I haven't seen an inductor of the size needed to balance a neutral.
More to the point, when you have a digital power supply that continuously outputs a 120V sine wave with respect to ground/neutral, there is no need to have a load balancing inductor/transformer, as the balance arises out of the power supply control circuits automatically.