As far as I know, backfeeding is illegal and not allowed by the NEC because of how dangerous it is to linepeople without a disconnect switch. A local transformer when energized by backfeed creates high voltage on lines that should be deenergized.
Pila can only back feed on-grid. Pila has a built in Microgrid Interconnect Device (MID). When an outage is detected, like Powerwall Pila opened the MID and continues to power downstream loads that are plugged into Pila.
My guess is the battery will come with some sort of remotely controlled switch that disconnects the house from the grid, before the battery starts supplying power.