Because the caption says "LETTER SIGNED TO FERDINAND IV, KING OF NAPLES, VERSAILLES, SEPTEMBER 24, 1788" (I am quoting verbatim, not shouting) rather than, for instance, the Proust example which is captioned "AUTOGRAPH MANUSCRIPT DRAFT OF THE OPENING PASSAGE, CA. MARCH–APRIL 1913" where autograph means written in the author's own handwriting.