Yes.  A logic which relies entirely on Keltham ignoring that logic and being ready to actually destroy Cheliax, because Keltham is Lawful, presumably the Pharaoh is Lawful, and obviously there are going to be truthspells involved.  So Keltham is, indeed, ignoring that logic, and proceeding as if he may need to destroy Cheliax at any random time in the next week, and not relying on Cheliax offering to do anything else which is not that; since, if Keltham relies on that offer, and ends up not prepared to destroy Cheliax, Cheliax will not so offer.

Keltham will go try to learn to cast cantrips within the Black Dome, then.

Yes.  A logic which relies entirely on Keltham ignoring that logic and being ready to actually destroy Cheliax, because Keltham is Lawful, presumably the Pharaoh is Lawful, and obviously there are going to be truthspells involved.  No?  So Keltham is, indeed, ignoring that logic, and proceeding as if he may need to destroy Cheliax at any random time in the next week, and not relying on Cheliax offering to do anything else which is not that; since, if Keltham relies on that offer, and ends up not prepared to destroy Cheliax, Cheliax will not so offer.

Keltham will go try to learn to cast cantrips within the Black Dome, then.