It relies on a legitimate government outside of it to provide actual protection from outside lawlessness.
A solarpunk society would decentralize power as much as possible in-line with Anarchist principles. Anarchist Catalonia (which had over 3.5 million participants) demonstrated that a decentralized society is entirely viable, and how it functioned was documented quite well (such as in this book). We also have a modern example of how Rojava (a somewhat decentralized society in Syria of 4.6 million people) does community defense.