Exactly. So many local first libs don't cover this that it makes me wonder if the applications I am typically working on are so fundamentally different from what the local-first devs are normally building?

Most apps have user data that needs to be (partially or fully) shielded from other users. Yet, most local-first libs neglect to explain how to implement this with their libraries, or sometimes it's an obscure page or footnote somewhere in their docs, as if this is just an afterthought...