Sorry that I have to disappoint you - this does only add par2 files to newly added files (and remove the corresponding par2 files when files are removed). So, for existing repos the par2 files must be manually generated for all already existing repo files.
restic (and rustic) will happily ignore additional files which don’t have a filename corresponding to a sha256 hash. But keep in mind that restic won’t add .par2 files to newly created files unless a similar solution is added like the one I’ve added to rustic.