I don't believe Mix3rz has ever claimed to have created the whole map himself and its quite clear that it is an edit of the Riverden map. He has also followed exactly the wishes of the original author in leaving in the credits on the nameboard and naming the map in the convention that they suggested.
Apart from that he has done a fantastic job editing and updating 2 versions of the map now and is working on a 3rd and the feedback so far has been very good indeed.