Man the Mitsurugi clan is evil.
If the demonkin where actually connected to the demongod and posed an existential threat there would be no reason to keep that secret.
You know why you would keep their power secret? Because their control of anima is better than a humans. So if they knew, or were trained, they could be stronger than the Mitsurugi clan.
Hell, their entire existence seems to be motivated not by defending the world, but by claiming or destroying every potential rival to their supremacy.
Anyone that awakens their anima is recruited or hunted down, anyone that leaves with knowledge has their mouth sewn shut...
These guys are just plain evil. And even worse they are hypocritical to boot. They sell themselves out to the empire and send their shinsou users out for political maneuvers. Despite all their rules about not doing exactly that. I hope Sora gets the chance to kill his dad and cause some reform.