This is true. Because .zip files are full files that you can run ingame on your server, which in the case of joining their server, you cannot run their Add-Ons. In Which Case, Badspot did it this way to prevent people from stealing Add-Ons, which if he set them as .zip Folders it would eliminate the purpose of private mods. The regular Folders are for allowing you to view them ingame at other servers. You have the Add-On, just not the full version.