Earlier quoted context omitted.
China could accuse Taiwan of developing nuclear weapons today, saying their spies uncovered the plot. Would you, I or anybody else be able to refute it? Would our doubt matter? And if Taiwan really did start manufacturing nuclear bombs and China was telling the truth about it, would you, I or others in the West believe them? The 'actual validity' of a casus belli has never really counted for much.
The difference is that, if they actually did start manufacturing the bomb, China would almost certainly be able to provide incontrovertible evidence. If they didn't provide evidence, yes, we coule refute it. The West will pretend that Taiwan wasn't doing that anyways. That's why I said 80%.
There is no such thing as incontrovertible evidence.