This app can only restore messages that were backed up by it before the messages were deleted. As long as you have a backup of the messages, it can restore from the backup. The contents of a backup can be checked using the Menu > View backups feature in the app or via the online viewer at this page.
This app only uses and supports backups in its specific XML format and cannot restore from backups created by other apps or in other formats like *.db, *.vmsg, *.csv etc.
It does not have access to backups that appear in the “Backups” section on the Google Drive app. Those are backups created by Google and cannot be used by other apps.