I suppose that's a retcon to "they just sent some random guy because the Demon Queen is a joke" gag at the beginning. No, they didn't. Soil apparently took this entirely seriously and sent their greatest butcher. I guess they always intended for Mirio to bring down the Kingdom after the Demon Queen so they could throw off the yoke of their oppressors. Impossible to know unless it's explained, especially since I don't think the author planned this out.