I can see that it is a bit unclear, but CIFS/SMB support is not really in Duplicati, it just uses the OS features, which on Windows allows the two first examples to work. The Linux (POSIX) file system does not work that way, so you have to mount it.
A better solution would be to include a CIFS client in Duplicati, so you could avoid having it mounted all the time (which would also be great for Windows users).