No, medical marijuana was legalized in California by Proposition 215.
SB 420 just further elaborates upon 215 by fixing certain problems it had, as well as outlining the rights of medical marijuana patients under state law. And yes, I believe they most likely made it SB 420 for that very reason.
Medical marijuana is not just for dying cancer patients. I have it, and I don't have cancer, nor am I dying. If anyone wants more info about it they can pm me.