That's mostly true if you are combining Powerwalls with Solar. If you are using Powerwalls as your backup electricity source (because you can't use solar for one reason or another), it's different. in either case you need enough Poweralls to charge your cars and in the non-solar case enough to last for several days.
Uhh... enough Powerwalls to charge your cars? So, SIX PowerWalls (13.5 kWh each) for a Model 3 (~75 kWh), for instance? That seems excessive. I think if the PowerWalls are acting as the backup power source, there needs to be a different plan for charging cars.