Do we only know about Julius Caeser and the Romans or any other ancient civilisation only through archeology and texts? or did people in the renaissance like Da Vinci or even people in the whole first millennium know about ancient civilisations? Are we the most knowledgeable humans about human history in the history of humans?
whilst I don't know of any sources regarding if Da Vinci personally knew of Caesar. I do know that Caecar appeared in Dante Alighieri's 'Inferno' in Limbo. So it can be assumed that people in the renaissance did have knowledge of Caecar and admired him enough to place him Limbo rather than a worst circle of hell.
I'll answer your last question with a yes, we certainly have the most knowledge of Human history.