Yes you're right, it's a simple network-like protocol allowing you to mount a path on the host OS to the Unikraft unikernel VM similar to a container volume. In addition, Unikraft's abstract APIs[0] allow for more block devices such as EXT{2..4}, etc. which you mount in a similar way. Alternatively, you can put your filesystem into a CPIO format and mount it as initram and load it into RAM (great for performance and read-only file systems, like webservers).
[0]: https://unikraft.org/docs/concepts/architecture/