Erm I don't know who's told you that but it's not right! Canon certainly produce their own lenses and there is a video
here which shows the Canon lens production facility and follows a high end lens in its construction.
Canon don't even give the details of the auto focus algorythms to third party manufacturers, they have to reverse engineer them, which gives some people concerns over the efficiency and long term compatability.
Certainly optically it is possible for third parties to produce lenses which are every bit as good as the Canon own brand lenses.