If he somehow, because Dunyain Fantasy, Kellhus internalized within the few seconds of exposure to the nuke its entire implications as a weapon (and therefore would extrapolate industrial capacity required, which is so immense that modern nation states find it challenging) then hes probably deep-diving some universal-truth-under-consciousness and therefore would know that he's lost.
If you can pull off nuclear restoration/enrichment then you can much more easily nail concepts like Production Lines and Mass-Manufacturing and apply that to your slave horde.
Nevermind that none of that makes a lick of sense, IF it happened it would probably cause Kellnus to get desperate enough to make a deal with the devil, intentionally, as its probably his only shot at winning.