This news isn't surprising, given the intensity of the Sino-Soviet border disputes/conflicts of 1969. The Nixon-Kissinger reaction likewise isn't a surprise.
Liu Chenshan, the author of a series of articles that chronicle the five times China has faced a nuclear threat since 1949, wrote that the most serious threat came in 1969 at the height of a bitter border dispute between Moscow and Beijing that left more than one thousand people dead on both sides.
He said Soviet diplomats warned Washington of Moscow's plans "to wipe out the Chinese threat and get rid of this modern adventurer," with a nuclear strike, asking the US to remain neutral.
But, he says, Washington told Moscow the United States would not stand idly by but launch its own nuclear attack against the Soviet Union if it attacked China, loosing nuclear missiles at 130 Soviet cities. The threat worked, he added, and made Moscow think twice, while forcing the two countries to regulate their border dispute at the negotiating table.
He quotes Soviet ministers and diplomats at the time to bolster his claim.
On 15 October 1969, he quotes Soviet premier Alexei Kosygin as telling Soviet leader Leonid Brezhnev that Washington has drawn up "detailed plans" for a nuclear war against the USSR if it attacked China.
"[The United States] has clearly indicated that China's interests are closely related to theirs and they have mapped out detailed plans for nuclear war against us," Kosygin is said to have told Brezhnev.
That same day he says Anatoly Dobrynin, the Soviet ambassador to Washington, told Brezhnev something similar after consultations with US diplomats. "If China suffers a nuclear attack, they (the Americans) will deem it as the start of the third world war," Dobrynin said. "The Americans have betrayed us."