China's nuclear capability in 1969 was extremely limited. DF-2 probably just reached IOC during or just before the 1969 conflict so it might not even seriously count.
Even at the height of the madness between the two communist powers, cooler and saner heads prevailed. The Soviets knew they could have nuclear carpet bombed China, but they also knew their 1 billion+ population was mostly rural. And truth be told, they had not enough nukes to anihilate every single chinese village.
Bottom line: had the Soviets nuked them, the chinese would have answered with their own bomb: the population one. Dozen if not hundred of millions of armed chinese militias would have invaded the Soviet far east to wage a hundred years guerilla. And against such humans waves, not even nukes would save the Soviets.
In fact Mao used his "population bomb" to corner both Cold War superpowers within the same decade.
In 1969 he threatened the Soviet far east with an invasion.
In 1965 he did the same in Vietnam, reminding the Americans of what had happened in Korea in 1950. Fact was that hords and scores of chinese had pushed back not only the Americans but the United Nations all the way from the chinese border to the tip of the Korean peninsula. Mao told Johnson he was ready to do the same in Vietnam: swarms of chinese across the north and then across the south, were the Americans were fighting.
In both cases the superpower leaders shitted their pants and backed down.
Although in the Soviet case in 1969 it went this way. In March 1969 the Chinese gave the Soviets a bloody nose. In August the Soviets waged a similar ambush, and got their vengeance. Both sides now had 150 soldiers killed in similar ambushes, and were licking their wounds. But the Soviets stopped right there: they knew the Chinese had a limited nuclear capability and an rather unlimited population bomb one.
Unfortunately the Chinese leadership was now scared to death and paranoid, convinced they had pissed the Soviets and would end nuked. Over the months of September and October, China prepared for nuclear warfare, going their own DEFCON 2 level.
Meanwhile Nixon was playing his madman theory with Kissinger, and together they tried to scared the Vietnamese, via the Soviets, by agitating their nuclear forces as if they were ready for a nuclear strike over North Vietnam. The latter was unimpressed, but the Soviets dealing with crazy chinese, now wondered WTH happened to madman Nixon. The latter was trying to put some "ambiguity" related to those B-52s bombing the heck out of Vietnam, kind of saying "be careful North Vietnam, we may drop some tactical nukes along the lot, if you don't return to the Paris negociations table..." North Vietnam (rightly) called it bluff, but the Soviets were a little worried if not paranoid.
Bottom line: early October 1969 nearly had a three way nuclear war between Madman Theory Nixon, Paranoid PRC, and baffled Soviets stuck between the two erratic and lunatic rivals.