If you are using a device that previously accessed your vault, it will be cached and accessible. It just won't sync until you regain network connectivity.
Is there an actual guarantee that all passwords will be cached (and not just e.g. the N most recently accessed ones), and that the cache will not expire at some point?
That failure can be mitigated by having a local backup, though. In general, with local stuff, it's not so much that the guarantees are better, it's that you can dial them in yourself to the level you feel comfortable at.