EU imports nearly 30% of their oil from Russia. Can China really take a bulk of that 30% - even at a discount? The Netherlands, Germany, Poland, Italy, Finland, and Slovakia buy a ton of their oil from Russia.
And if Russia doesn't have as many buyers buying as much oil, they'll have to decrease production which would also decrease world supply. And if they have to keep selling at massive discounts- it could become too expensive for them to keep producing the oil.