Come on now, they went to laugh tale and learned the " truth of the world " Rayleigh said they could've misunderstood some things but they 100% know about Imu's existence, now why they didn't told anyone?
Well Oda has to do some explaining ofc they all seem to hint that something particular has to happen in the current time frame that will trigger the giga war, Gaban even kinda expected Elbaf to be potentially destroyed or in great danger like quite imminently.....
He was also able to stop Sommers's regeneration which whether it's just Haki or something else ( I'm still hoping for something else than just Haki xD ) it means he's knowledgeable about Imu's powers or at least knows how to deal with them, probably learned about it during god valley, saturn and the HK where there after all....