Not a mistake!

It is a case of expletive ne that non native French speakers often confuse with a double negative.

A real double negative would have been:

Moins que jamais, ces pays ne sont pas prêts…

Moins que jamais (less than any other circumstance) is the negation of the adverbial expression plus que jamais (more than any other circumstance, i.e. more than ever). There is no less than ever in English though.

The ne in ne sont prêts has no strong negative meaning. The sentence might have been written:

Moins que jamais, ces pays sont prêts…

but that would have been less elegant. Moins que jamais is kind of a shortcut of aujourd’hui moins que jamais so another way to write the sentence that provides an explanation to the ne presence can be:

Aujourd’hui, ces pays ne sont pas prêts à renoncer à une intégration nucléaire, et c’est encore plus le cas que ça ne l’a jamais été.

Should have the sentence started with plus que jamais, it would have read:

Plus que jamais, ces pays sont prêts à renoncer à…

the opposite of the original meaning.

“moins que jamais” is clearly not a common use for “jamais”.
it might not be a mistake on the grammatical side, but it sounds terrible.