The disparity of military strength and power between and China and Soviet Union in the late 60’s was alarmingly great. The Soviet attack was averted because China was prepared for the war in that the whole nation was mobilized in war preparation such as digging tunnels, storage of foodstuff, and secret denotation of a nuclear bomb, etc.
Looking back many people today laughed at those crazy war preparations of that period. They failed or didn’t want to recognize the war was avoided because of the “crazy” things.
Yes, the US opposition was crucial but not the determining and sole factor. The US would not come to the aid if the Russia attacked. At best the US would remain neutral. Just my two cents.