Look at a map, China doesn't have open access to the ocean. They have Japan, Taiwan in their way, the only help out they have is the South China Sea, but even that is crowded and relies on Vietnam, Philippines, Indonesia not being again them. They can only really get oil from the middle east through the straight of Malacca (see https://en.wikipedia.org/wiki/Malacca_dilemma), which is really easy to cut off. It doesn't matter if you can build lots of ships if the US navy can take them out as they leave port into the open ocean.

So overland is their best option, but Russia is huge, lacks infrastructure (no oil pipelines in the far east), and anyways, isn't always China's friend. Western China abuts some Central asian countries with oil (and Iran in the middle), but again, not so friendly, lots of mountains to get trains through, and it is still far away from East China where the oil is actually needed.

So...they have lots of coal and are really pushing into renewables and nuclear. Nuclear is the kind of thing where you can stockpile a few thousand years worth of fuel with a few ships from Australia in peacetime. And if your economy relies more on electric trains and cars, you are going to be ok if someone cuts off or restricts your oil supply.