Manual of the Planes 3E has Kiriansalee disappearing shortly after the rumours of Orcus returning. Either Orcus has killed her, or she's afraid enough of him that she's gone to ground. Bottom line, he's a threat to her (and how could he not be? He killed Bwimb, Manzecorian, Primus, etc.) and Orcus likely has his layer back.
This would seem to imply that Divine Rank is not an automatic trump card in combat, and beings that do not necessarily have any intrinsic Divine Rank can still kill those that do if sufficiently powerful.
Given what's presented in Deities and Demigods, it seems that short of a deity having Annihilating Strike, there's nothing that says a god of higher Divine Rank is always capable of defeating those of lower Divine Rank in combat. Certainly each major category (Demigod, Lesser, Intermediate, Greater) has some advantages, but lacking the raw combat stats to back them up, they can still be killed.
Ergo, divine beings, like Kiriansalee, are quite justified in being frightened of Demon Princes and Archdevils. Overfiends (to invent a generic term) may not have the power over the Prime Material that deities do, but they're quite capable of defending the layers they hold in their respective planes, otherwise they wouldn't rule them.