As difficult as it is to prove a negative, the Airfacts Journal seems to make a convincing case of identifying the actual author.
The site’s writer has been dubious about the quote for many years and carried out much reading of Leonardo da Vinci’s notebooks and recorded writings trying to find the line, despairing that it may be in some largely unpublished ...
The quote is from Chapter Nine of The Paths of the Dead, the first volume of The Viscount of Adrilankha, by Steven Brust.
Piro shrugged. "I have no such plans, but, whatever the Count's plans for me may be, I have no intention of staying here for-ever."
"Ah! My lord is ambitious?"
"So much the better."