What cope?
Uranus destroyed Lulusia (and God’s Valley, presumably). Not Imu. The most Imu will be is a counterpart of Shirahoshi that can control Uranus through their bloodline, hence getting to sit on the throne. Not a frontline fighter.
There’s no reason for a God Tier Imu who also has control of an Ancient Weapon to hide their existence from the world they rule. There’s no reason for a God Tier Imu, their five top tier Gorosei and an Ancient Weapon to even bother having Marines. Saturn needs an Admiral bodyguard and a food taster, this guy is meant to be a top tier?
Far and away the most likely answer- Imu’s family line can control Uranus, so the other Celestial Dragons sit them on the throne. They can‘t actually fight though, and the Gorosei et al are shit scared that someone like Rocks, Blackbeard or Kaido finds out Imu exists, kidnaps them and can control Uranus through them. Hence the “King of the World” being basically a prisoner in their own castle and the greatest secret of the Celestial Dragons.
God’s Valley showed the end of One Piece, and Roger decided that Rocks was a bigger threat than the Celestial Dragons. Blackbeard will be the modern day Rocks. He’ll succeed where Rocks failed, kidnap Imu and make himself King of the World.