Happy Thanksgiving @jafant!
I can speak only from intuition, as Cooder didn’t speculate. It is my suspicion that Brian Jones was responsible for more of The Stones direction than he has been given credit for. You’ll notice that it is Mick Taylor, not Keith Richards, who is playing the signature guitar parts on the albums he appears on.
Ry Cooder was brought in as a session guitarist, not as a source of material. Apparently Keith was at a complete loss as to how to play his own songs. Have you ever known a junkie musician? They are basically worthless. Mick rolled the tapes while Ry played, then Keith came in and learned Ry’s parts. Pathetic.