Authors never wrote their most famous quotes?

Michael Thompson points out in the post linked above that Luther probably never said these beloved words:

If I profess with the loudest voice and clearest exposition every portion of the truth of God except precisely that little point which the world and the devil are at the moment attacking, I am not confessing Christ, however boldly I may be professing Christ. Where the battle rages, there the loyalty of the soldier is proved. To be steady on all battle fronts besides is mere flight and disgrace if he flinches at that point.

And Voltaire never said:

I may disagree with what you say, but I will defend to the death your right to say it.

Francis never said:

Preach the gospel. If necessary, use words

and never prayed

Lord, make me an instrument of your peace

.

And worst of all, Bach never wrote his two most famous minuets in G, nor his lovely musette!