The way i see it, trade benefits China in 2 ways.
The first way, obviously you buy and sell goods needed and you get or give money, thats simple stuff and has been happening since people started trading.
2nd way is that with for example buying a lot.of stuff from a country you can also buy influence there. So now that Australia is restricting investments, is going to media war with China, throwing to jail Chinese reporters etc, it means that China doesn't gain any influence in Australia.
So why waste your time and money buying something that exists in many parts of the world from Australia and you can only get money traded and not influence?
Thus it is better for China to buy from Russia (agricultural products) to get it closer to the China's sphere. Better to buy more coal from Indonesia, better to get iron from Brazil to break it from the US influence, plus they have the phase 1 deal with the US so why deal with the vassal when you can deal with the Lord
And now China is in a better situation it can go to ASEAN countries and say look, I am not spending 10 Billion per year on coal from Australia anymore, what can you offer me so you can persuade me to buy coal from your country? The same applies to Brazil, African countries, Russia and many others.
Why waste money in Australia where you cant influence them, better spend it somewhere else to get them closer.