score:5
There were quite a few much heavier constructions that have in fact been build and moved by manpower. Helepolis was probably the biggest one, estimated 160 tons, and it was build and moved and used as intended in the siege of Rhodes.
If the speed is not an issue one can move very heavy things with pure manpower. Archimedes famously used pulleys to move a ship single-handedly. And most modern drivers, who are much weaker than 15th century soldiers, are perfectly capable of pushing a 2-ton car that wouldn't start.
Leonardo's tank was a viable design IMO (apart from intentional trivial errors such as reverse direction of some screws). Since the tank didn't have to protect against 20th century artillery its armour could have been much lighter than in modern tanks. Since the goal was to get the crew to the destination through enemy fire at any speed rather than hurry their full throttle there was no expectation of high speed. A dozen or so man were perfectly capable of pushing a wheeled contraption under 20-ish tons.