I'm wondering if it's maybe not so much the distinction between freeholding lords and supernatural nations, as it is pure power.
It's a bit unclear to me what the Unseelie Accords really mean in this case - but the challenge Harry levied may well have been more or less 'respond to our challenge or face war' rather than what we assume 'respond to our challenge or be in violation of the Accords'.
So, Gard might fear the Denarians could just shrug off being challenged by Monoc, feeling they could accept their emnity (and, at worst, being declared war on) while they could not afford that even with a distracted White Council (who are, notionally, the lynchpin of the supernatural balance of power). And Nick would know that and act accordingly.