I'm not certain what part of "In fact, Dulles was never authorized to make such an offer and there is no hard evidence that he did so." is not clear to you, but no hard evidence is what it is.

Quote:

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.......using two or three tactical nuclear weapons against these enemy positions".

Bidault declined, he says, "because he knew… that if this killed a lot of Viet Minh troops then it would also basically destroy the garrison itself".

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'Tactical' NOT 'strategic', which are commonly referred to a 'city busters'.

So if Dulles was casually throwing around a 'suggestion' it wasn't viewed as practical.