Earwag - I think the nuke tipped off Kellhus. The Inchoroi used nukes in the wars against the Nonmen, as recorded in the Isuphiryas, as Kellhus mentions, but we haven't seen anything like them in the Sagas. I gather the implication is that the Consult either couldn't make the nukes work, or the Inchoroi had run out of them by then. So in light of that, Kellhus came to the conclusion that it has to be more Dunyain, as who else would be able to either rediscover how to make the nukes work, or to build more?

Granted, the timing and multi-purpose function of the nuke might factor into it as well: deal a terrible blow to the Great Ordeal while also destroying the main food source of the survivors. Not exactly rocket science, but maybe more strategic than the Consult had been in his estimations up until then.