Deng Xiaoping's 1975 quote was not about US or USSR monopoly on nukes... more on Indian monopoly of nukes on South Asian subcontinent. The Deng Xiaoping quote was dated in 1975, just one (1) year after India's first detonation of nukes in 1974, and 4 year after Pakistan's dismemberment in the civil war and Pakistan's loss in
Indo-Pakistani War of 1971.
Deng Xiaoping allegedly granted help to Pakistani nuclear program in 1982 to offset India's first nuke test in 1974. Pakistan was recieving Chinese nuclear scientist exchanges as early as 1976.
So in China's calculations, if there is a nuclear monopoly in South Asia, then arming Pakistan with nukes is reasonable to check it's joint chief geopolitical rival (India).
So it's reasonable in China's calculation that if there is monopoly on high-tech (non-nuclear) conventional weapons on Korean peninsula (SK) , then accepting NK nukes will prove an balance of power stabilizer to SK's overwhelming tech advantage, and US overwhelming tactical nuke advantage.
That is your opinion, That is not the opinion of the Chinese President.
This is what Chinese President Xi Jinping said to the South Korean President Moon Jae-in during the G20 summit:
Source:
This suggests that even if NK launches an ICBM or performs a nuke test, Beijing will do little to hurt the alliance. Xi Jinping basically told South Korean President that China would never abandon North Korea and never accept a pro-US unified Korea on her doorsteps despite nuke and ICBM tests.
See above quotation,
Chinese President Xi Jinping said the fundamental "blood alliance" relationship hasn't changed despite nuke and ICBM tests, and this wasn't to a domestic audience, it was to the Top #1 leader of South Korea, Moon Jae-in in a bilateral face-to-face meeting.
The policy implications of telling SK President of the "Blood alliance" in his face cannot be underestimated, since word choice is highly symbolic and important in Chinese diplomatic circles.