It does count toward the $10 minimum monthly bill so for people already at the minimum bill it only adds $6 a month. Obviously this calculation only matters for those thinking of adding an EV since if you're already at the minimum bill then your rate plan doesn't matter.
Adding more solar is probably more economical than powerwalls right now but I wouldn't be surprised if solar becomes even less valuable in the future. It feels like we're in a utility death spiral in San Diego. SDG&E wants to raise the minimum bill to $38 a month.