I believe the advisory panel's position is that it is not indicated for the treatment of Alzheimer's and that Biogen should not be allowed to say this.
The FDA could have said "this drug is safe and may be sold, but you may not describe it as an Alzheimer's treatment, and you must explicitly state that it has been found to be ineffective as an Alzheimer's treatment in clinical trials".
So how is Biogen going to get doctors to prescribe and people to pay $50K for a drug with no brand recognition nor eligibility for insurance, and no claims of actual effectiveness?