It is nearly 40 years since I first heard that story from a British China-watcher in Hong Kong. Sometime soon after that, British Kremlinologist Robert Conquest mentioned it in print.
I think it’s quite probably true. Sino-Russian relations were very fraught in 1969. The Soviets had for years had a general impression that Mao Tse-tung was nuts — see Khrushchov’s memoirs. Mao’s Great Proletarian Cultural Revolution, which started in 1966, can’t have done anything to diminish that impression.
I have heard that a strike by the U.S. was also discussed at quite high levels in the mid-1960s. The Chinese nuclear-testing facilities at Lop Nor in East Turkestan were already visible on satellite photographs. We could easily have hit them with a barrage of ICBMs. There would have been almost no civilian casualties. The area is so remote, in fact, we could plausibly have denied being the cause of whatever the ChiComs said happened there.
Perhaps our grandchildren will curse us for not having taken the opportunity.