If you recall, Overlord did mention that not everyone replied publicly. Therefore, it may be that he is indeed following his own rules set forth in the ultimatum, but we just don't see the whole picture.
I asked, as the first to reply, that others reply publicly such as I.
The only reply we did not have publicly was from Rio. They say that they replied privately 20 minutes after the invasion "as we bickered like children". I don't remember Overlord specifically asking for a public refusal. Mind you, some could have refused publicly and sent private "ignore what I just said" messages. Old Grehk's declaration was pretty timid, for example, but Overlord's speaches don't tend to make me believe they were the ones (though I guess it's possible).
It is possible, however, that Overlord overlooked one message, or is reacting to Enweil's being late (Overlord said the target had been picked 7 hours after the ultimatum was made). Overlord said:
You fools fall for the most primitive piece of misdirection, because you can't trust each other. It is true, not everyone replied in public, but I never said everyone had done so. Someone amongst your kind knew full well that he hadn't yet replied when I announced the winner of our little game.
And he kept quiet. I can not believe it! He left his own kind to be crushed. Did he think I missed him and didn't want to point it out? Or was he secretly happy?
So from what I have, I'd have to say Rio just lied. Because Enweil did not "keep quiet" about the ultimatum. Though I guess you could say they "kept quiet" about not having been targeted, despite replying late. Besides, even had anyone else been the one to declare late, I wouldn't have expected them to say "No, I was late, pick me instead", because that would be recognizing the daimons' legitimacy in attacking whoever saw the letter last, admiting that the late realm was "at fault" and that he should be the "rightful" target. Not to mention that Old Grehk is probably the most easy realm to defend of them all, being pretty damn intact and surrounded by strong allies and other human realms as opposed to Enweil which was badly damaged in the invasion and is just surrounded by Fheuv'n (tiny and poor) and Rio.
Guess it could be Enweil, though. Or Rio lied. Or someone sent a "ignore our public annoucement" message. We'll see soon enough.
I just hope we get to TO Zuhle before they strike, though... we *really* need to shorten the gap between ourselves and Fronen for these food transfers.