Honestly, my guess would be that Imu is not of the One Piece world, or at least cut from the same cloth as Joy Boy. When Imu killed Saturn with smoke, all Imu was pissed about with Joy Boy escaping Egghead, which Saturn did by not pursuing Luffy.
Meanwhile, the Gorosei are the ones who killed Clover before he said the name of the kingdom (or at least authorized everything that let up to it), are the ones who know about Luffy’s devil fruit being named for the Sun god, Nika, and we’re pissed that Luffy’s new wanted poster had him in Gear 5 and try to stop it getting released.
So Imu gives powers to the Gorosei in exchange for worship and allegiance, but doesn’t tell them about Joy Boy, and there may be a chance that the depths of knowledge the Gorosei have about the Void Century may be as limited and scattered as the average One Piece reader, only completely connecting and explaining the endgame once Luffy comes across this information (and sleeps through it or it goes in one ear and out the other. Or doesn’t care in the slightest.)
In the end both the Gorosei and Imu are doing the same thing: fighting feverishly to preserve the status quo from an agent of chaos harnessing what seems to be an unstoppable force in the form of willpower. Except Imu has been doing this for longer and on a grander scale. And I can only assume knows the true nature of the chaotic adversary, and may even keep this info from even the closest of followers.