DaVinci was brilliant and his ideas were ahead of their time! However, just because someone can draw an idea does not mean it was practical, workable or implementable, or for that matter possible. (see his helicopter) There is an individual where I work that is always spouting some advanced new technology he has developed recently. They always seem to prove out well known theories such as Ohm's Law or such. My 9-year old also draws wonderful fire-breathing hot rods that can fly, but I doubt they will change history.
DaVinci probably has received a significant amount of credit for some some simple drawings that may have been rehashed from previous historical war machines. I have no proof in these matters but early civilizations don't receive near the credit they deserve for their technological development either.
I just think DaVinci's designs, drawings and sketches need to be taken with a grain of salt, that's all.