--means that the following input cannot treated as parameters, so just file paths in this case) you can cause an arbitrary error to read a file, so if a command like the following one is being executed by root:
@root.txtand the file
root.txtbeing a symlink to the file you want to read:
root.txtas a file containing the list of files it should compress (thats what the existence of
@root.txtindicates) and when it 7z read
root.txtit will read
/file/you/want/to/readand as the content of this file isn't a list of files, it will throw and error showing the content.