Earlier quoted context omitted.
Charging or dumping a single powerwall will not damage the grid. However, if someone causes many of them to do so in sync, the current grid control systems are definitely not going to cope well with that sort of behavior.
To do that the attacker would need to connect to hundreds (thousands?) of Powerwalls at the same time and figure out the password for each of them. Which means being close enough to each of them to access the Wifi and also figuring out their serial number somehow. How would they do that?
One may hypothesize about the feasibility of a router-targeting worm (similar to the Moon router worm, targeting Linksys kit) that has a payload that searches for nearby PowerWalls and uses its owners's resources to play guess the password. Getting in wouldn't be fast, but over a period of months such a worm could well gain control over thousands to tens of thousands of powerwalls. At which point, a DDoS attack on national-level grid infrastructure is within reach.