So is the author suggesting that dinosaurs were around at the time of da Vinci?
Jordan Peterson would say that man’s created mythologies (such as dragons) are hard-wired into humans by very deep biological and psychological processes, perhaps for our daily survival as a species, but also perhaps for some deep metaphysical explanation or representation of our existence and justification for being.
By that logic, Da Vinci never saw a real dragon, but the concept of them exists within all of us
No, he is taking a sketch Da Vinci made of a dragon. And claiming that this proves Da Vinci was wrong about what dinosaurs looked like. And that proves that if he could be so wrong, that all of us are also wrong who shake our heads at the 4500 year old earth.
Several leaps of faith in there. The first being that Da Vinci was actually attempting to accurately show us what a dinosaur looked like.