Coal is (mostly) a commodity (there are some differences in quality of coal, which means that if China substitutes Australian coal for coal from other locations, Australia would then sell coal to those who would have bought those alternative sources of coal. There are some inefficiencies by doing so (e.g. buyers are now buying from further away locations so higher shipping costs), but the net effect has not been as great as it has turned out to be.

Australia is still selling all of its coal,and China can still purchase the same amount of coal as before, just from different sources.

The monkey wrench is that the demand for energy has increased greatly and the worldwide supply of coal has shrunk, and thus coal price has greatly increased for everyone. That is great for Australia, bad for China, but has very little to do with China's ban on Australian coal (which only marginally hurts China)