The thing to remember about da Vinci's drawings of tanks and flying machines is that they are just that: drawings of conceptual ideas. He was wondering about how he could possibly make an armored car and sketching out his ideas. If he had seriously believed it practical, he probably would have pursued it further. Also, ANY mind is capable of making a simple mistake.

On that end, a tank is VASTLY different from da Vinci's armored car. The only similarity they have is that share the idea it is useful to put armor on something.