I've only ever seen that in Wikipedia (unless it was in Rosebery's preface and I forgot), but yes, that is what I've been told. So you're right, especially if he was composing/editing after 1784 (i.e. the Voltaire memoir year), he was getting pretty close to blindness years, and he might well have run out of time. I mean, he probably could have gotten an amanuensis, but it might not have been his top priority.

But seriously, scholars need to call more attention to the visible seams and general unreliability of these memoirs.