I thought it was strange because 1) he keeps saying he sees Nau Cayuti and that he's speaking to him 2) even though NC is nowhere to be seen and he thinks it's Gilgaol coming to him and speaking and 3) the being is described with four horns like Ajokli.
I suspect the Second Apocalypse won't be like the first and that it's something Kellhus will bring about. Even though Achamian prods him at the end of TTT, saying that he speaks of an Apocalypse when claiming all things are rewritten with him, this is a hint that it really will be an Apocalypse when everything he's planned is brought to fruition.
I think with this Celmomas perspective of the Prophecy, it seems clear that Celmomas segues without saying so and that as Seswatha, as the reader, we've previously not perceived the segue and so continued to interpret Celmomas as speaking of Nau-Cayuti.
To the second point, there's a good chance the monicker of the series actually uses apocalypse as a stand-in for "revelation."
Arguably, we could be incredibly over-the-top about the appellation and assume that Bakker is suggesting that
The Second Apocalypse is actually The Second Enlightenment... you know, like the Enlightment, which would subsequently be called The First Enlightenment

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Well, just to make it more confusing, both Ajolki and Gilgoal are described as having four horns. I don't think we ever see Gilgoal any other time. I'll have to go back and see if any of the idols of Ajolki ever give a further description though.
And, during the scene it says it's Gilgoal, correct?
Celmomas perceives it to be Gilgoal.
I have to say, after reading the actual book finally and reading the speculation here and at Westeros, I'm inclined to assume that since it's perceived as such an obvious catch that Gilgoal is actually Ajokli, not that Ajokli is Gilgoal.
I'm not sure why that distinction matters to me but it does...