You misunderstand. The cartel held a monopoly. The cartel comprised of multiple companies with multiple patents. However you want to phrase the end result, the

You misunderstand. The cartel held a monopoly. The cartel comprised of multiple companies with multiple patents. However you want to phrase the end result, the cartel could only keep out competition under the legal threat the patents provided. Eventually competition was able to come in and break up the cartel, but without the legal monopoly provided through the patents, the cartel would have never lasted as long.