^I think that scene shows foremost that the Founders think they can hand out promises without necessarily keeping them - probably because they think they are so far above solids that promises to them don't really count, or some such thing.
The female founder decided on a whim that the entire Cardassian population had to be exterminated, a people they were allied with up until that point, just because their military switched sides. So why such a decision require any more deliberation on their parts, or what would make them have any compunctions, when it came to eradicating Earth?