Where does Canon say that? AFAIK (and confirmed by Photonotes), the 300/4L (non-IS) works just fine with all the Canon EF extenders (although with the 2x TC you're at f/8 meaning no AF on most bodies).
This is what Canon Canada said : "Dear David
Thank you for your E-mail inquiry regarding your Canon lens.
I am sorry, the lens you are referring to is not compatible with any of our EF Extenders1.4x or 2x.
Should you require further assistance, please feel free to email us or visit our customer support website at http://www.canon.ca
Technical Support Representative
Customer Information Centre
Canon Canada Inc. "