Yes, this is spot on! It is very hard and exponentially harder as scope and audience increases. I have experience with this too for Humane Tech Community: when we first adopted the humanetech.com vision of ‘creating a cultural awakening’ our scope was all tech and our audience was the entire world. In any case community-building can easily be a full-time job (it was for me for a long time as a volunteer, but now I am just casually involved in HTC).
For Solid Community both scope and audience need not be that large and mission not that ambitious. Audience are mostly tech-oriented folks (techies) working on their pet projects and interested in linked data and bringing Solid to that. So scope is ‘solid-compliance-in-my-project’. And the mission need not be ‘reboot the semantic web’ at all, though the vision of The Decentralized Web is where all of our passions are.