If that's the case, why have they been getting along so much better with each other since 1993 than with the US?
Because the ideological element has been removed, and the balance of power has so shifted in China's favor they don't think they need to use force. Russia continues to supply China with energy, and military technology. The border areas are under Chinese economic domination, and the Chinese are just settling in the towns of the region. The Russian population density is lower then in the State of Wyoming. China could just walk in anytime they want to, and there would be nothing Russia could do about it.
Russia is running a bluff, that if anyone crosses their borders they would immediately use nuclear weapons. That's a cry of weakness, saying they can't defend themselves with conventional forces, but it's a bluff, because using nuclear weapons to defend lost territory would be suicidal. So the relationship continues with China in the dominate role, and setting the terms. Russia is a declining power, that poses no threat to Chinese interests.
China's power relationship with the U.S. is very different, it's more reciprocal. The U.S. is still militarily, and technologically superior, and China is surrounded by U.S. Allies. China sees the U.S. as a threat to it's ambitions, but it needs to have friendly economic relations. So with Russia removed as a threat the Chinese can concentrate on building up their naval, and airpower, while using their now smaller, but better trained, and equipped army to face India, and Vietnam, while still threatening a plausible combined arms invasion of Taiwan.