Why would China put themselves in the thrall of Russia when they are often hostile towards one another?
Because Putin is a moron who is not only supplying China but doing so on the cheap so he can credibly threaten Europe with cutting them off for the Winter without destroying his own budget and economy.
The threat from Chinese immigrants in the Russian East is far greater than any threat from NATO but there we are.
And China is on a Nuclear power plant building boom too.
When a seller is as desperate to sell as a buyer is desperate to buy neither has a huge advantage.
With increased sales to China my guess is Russia will become dependent on that extra income as well.