True, but while vanilla mortals aren't signatories, other groups do indeed have human members. I suspect Monoc Securities has more than a few employees who still count as human for First Law purposes, and I'm sure several other supernatural nations do as well (thge Summer and Winter Knights come immediately to mind), so while White Council members aren't going to duel each other under the Unseelie Accords, a human vs. human magical duel is still a possibility.