It doesnt need to completely disconnect from the grid, just smooth the load as much as is economicallly useful. A load that is already statistically smoothed by multiple cars charging at once, arriving and leaving at different, but statistically predictable, times.

And people are already attaching batteries to the grid to earn money by smootging supply and demand, it just makes sense to locate those near to demand so they can share cables. Then to the grid the car charger station looks like a consistent energy user that can flex a little to help the grid (possibly even feeding back during peaks), rather than a random one.