A Google search for “all-versions” at site:duplicati.readthedocs.io supports my hunch, which is that “search” means the FIND command, which is how one searches. You can try that at Commandline.
“Use this option to show all previous versions too” text suggests a find, which is also called list.
The RESTORE command also has a syntax which seems to say multi-version restore isn’t allowed:
--version=<int>
Restore files from a specific backup.
https://github.com/duplicati/duplicati/blob/master/Duplicati/Library/Main/Operation/ListFilesHandler.cs
looks like the only place AllVersions is used, so I don’t think it affects Restore, however I’m less sure whether Restore from command line can be persuaded to do multiple versions using a –version list… Such a request could be problematic. Which version wins if a file is in several (unlike in original post)?
https://duplicati.readthedocs.io/en/latest/04-using-duplicati-from-the-command-line/
shows commands where --version is specifically said to do multiple versions. Restore is the opposite, which means my guess is that you won’t be able to persuade it otherwise. You could certainly test it…