True, that part is vague, but there aren't many things that "threaten the world by their very existance", as he states right afterwards. The primals do, due to their aether-consuming (after all, aether and crystals are what the Garleans are intersted in, too), and the Ascians do as well due to their very nature and also indirectly aether-consuming plans, scheming and helping with summoning stuff. (after mog, you can't really call it "monsters" anymore)
Dragons surely are a vicious, mighty, fearsome race for someone with a control-fetish - but I don't think the Garleans hate them enough to proclaim such a rigorous statement.
