I figure these addon's are, as IJS stated, loaded from the server onto your pc. And are only loaded when they are on the specific-server. Thus you don't need to download any modifications/addon's at all.
Compare it to the Warcraft III-mod for Counter-Strike. When you join a server with this mod you only have to download all the .wav-files and nothing more to play this mod. Even though it is a huge gameplay-change.
So to go short: Modding will only be allowed via specific server's and people can only use their own mod's by making their own server's.
Sounds very, very good to me.