As far as I know, the storage:link creates a symlink from the public folder to the storage folder. But FTP only supports uploading regular files and directories, it doesn’t support symlinks. So either the folder is missing from your website, or it contains a copy of the contents of the storage folder as it was before uploading.
Unfortunately, there is no way to create symlinks on our hosting (the PHP function to do this is blocked too). So this method of exposing storage will just not work on free hosting.