It is quite possible for there to be an asteroid that is composed of more gold than all of the gold that has ever been mined on the Earth in human history. Gold is produced in the core of a supergiant star as it explodes in a supernova or hypernova, spreading the gold vapors into a nebular cloud in space. When gravity and other forces cause the cloud to condense again to form another star, planets, and asteroids, there is an opportunity for the gold to become concentrated into those bodies in space, including the asteroids. Imagine what the discovery of a gold asteroid within reach of a space mining company on the Earth could do to the Earth’s gold market. The same is true of platinum.
You're right. I read that all the Earth's gold found so far is smaller than an olympic sized swimming pool, or something to that effect. I imagine there is much gold on the Moon, as it probably is made of many of the same minerals as the Earth. Asteroid hits on the Moon may have scattered gold around the surface.