IMO, "cd", "grep", "cp" and etc. are provided by shell and os, not by file system. They are just human interface to help you interact with the underlying file system. Can you call "cd" on an ext4 file system directly? No. The shell will invoke "cd" or similar process and that process will do the system call to kernel, and then kernel will call the ext4 kernel module through VFS.

https://www.kernel.org/doc/Documentation/filesystems/vfs.txt

For Zbox, it is just like a kernel module but runs inside application memory space. To interact with it, you have to use a set of 'standard' API, in kernel that is VFS, in Zbox that is Rust's fs API (not exact same, but similar).