Their argument hinges on the fact that the cannabis plant itself has not gone through the process of FDA approval for the treatment of a disease. I think every drug on their schedules 2-5 has either gone through this process or (for older compounds) has an uninterrupted pattern of official medical use in the US.
That argument, of course, is somewhat apart from the discussion of whether or not cannabis should be de-scheduled. Descheduling is potentially much faster than medicalizing (as with the 21st amendment).
Also, if the FDA approves Sativex, it essentially approves a strain of cannabis.