It's weird how Rovagug cultists are still a thing, given that you'd expect the gods to cooperate on stomping them. All mortals are just opaque to gods unless the gods spend lots of effort? Why not pay Nethys to tell you about them?
Suppose, though, that something about Rovagug isn't just a Prophecy blindspot, but some sort of greater attentional blindspot for the gods.
Also in terms of trying to read ahead in the plot, Asmodeus letting Rovagug out of Its vault, under some circumstances, suggests that Rovagug can be directed, possibly, yes, in exchange for being let out of its vault?
Keltham is not doing that tomorrow morning, very very obviously.
But he is setting up possibilities in advance for moving to within one alignment step of Rovagug so Keltham can be its cleric. If, to be clear, that later starts to look like a good idea.