There are red lines of rhetoric and red lines of substantial fact. In realpolitik or international relations, threatening the security of a foreign state, especially its ruling class, is going to be a "red line" whether it is announced as such or not. Clearly, the weakening or fall of Russia would be extremely disadvantageous to China, that is, the US would be on China's back like the hungry tiger it is. Therefore, China will support Russia whether they say they are doing it or not. They would be crazy not to. In fact, I would be very surprised to learn that China has not been sending support to Russia already and to have big plans for increasing it should it seem advisable. The more the US threatens China, the greater that assistance will become.