When accounting for air resistance, heavy objects do fall faster than light ones. They couldn't test in a vacuum back then, they only knew how things work here in Earth's atmosphere.

A similar size chunk of iron and coal would have done the experiment just fine. Any two objects of the same shape and size but significantly different densities.

If two objects have the same size and shape, the force applied by air resistance will be the same. However, if two objects have different mass, that same force will result in different acceleration.

So change the shape, a long copper rod and clump of coal.