nbackup's documentation explicitly allows the use of aliases for backup.
However if aliases are used exclusively, that is with DatabaseAccess=None, nbackup will break.
nbackup still manages to get the right filename (propably attaching to the database using the alias) and is still able to access the database file (at least in my tests running as root), but seems to access the database later using the absolute file name, which then is rejected by the server.
nbackup should use the alias to connect. It would then even work with DatabaseAccess=None.