That Gas.
THAT Gas - the gas that was going to Europe.
If you have two houses, one in Florida, and one in California, that get their water from local pipelines, and one gets cut off, there is a problem in assuming that you will just use the water from the other house. They are not connected. Running a hose is not quick or simple, and probably not worth the cost.
Russia is much bigger however, than just the distance between California and Florida (11 time zones wide).
THAT gas (from European pipeline networks) cannot magically appear from spigots in China, just as you can't just turn on the California water at your Florida house. Pipelines would have to built, to move THAT gas, or THAT water across a continent first.
The pipeline that does exist between Russia and China (Power of Siberia) connects smaller gas fields in the Far East and Sakhalin Island to China - not THAT gas from the huge Arctic Yamal fields, or the Caspian Basin, that (used to) service Europe. There is nowhere else for the great bulk of THAT gas to go, except up in flames, or cap the wells.
Pipelines seldom operate at peak capacity - peak pressure - all the time. While China is wanting to buy more Russian gas (as they have in the last few months), they have some margin to increase pressure, to move more volume through that (Power of Siberia) pipe. Lately they have been pushing near peak capacity through the Eastern (Power of Siberia) pipeline (roughly doubling last year's throughput rate) - but this has nothing to do with THAT gas, that used to go to Europe. THAT gas is stuck on the other side of the continent, around the bend of the curvature of the Earth, far, far away. THAT gas, that used to go to Europe, was 85% of Russia's gas exports last year, running through a huge network of pipelines, built over more than half a century.
OK, for the second time, I read your link about a "new" pipeline to China. For the second time, I will quote that article, and point out that this proposal is for the distant future - 2030 or beyond.
"Designed to carry natural gas to China via Mongolia, the Soyuz Vostok pipeline is part of the larger Power of Siberia 2 pipeline."
Power of Siberia 2 was planned to be built over 1,600 miles from the Arctic, to bring gas to China. The pipeline in the article you linked (Soyuz Vostok) is envisioned as a spur off of that proposed long haul pipeline (which does not exist, was planned to start flowing in 2030, but had its financing pulled when partners withdrew this year, because of the invasion).