Admirals do not have the mark of Abyss, Imu cannot just easily kill one of them as he/she did with Saturn.
Rocks vs Admiral is public information from the Revery, I doubt it happened away from all other eyes.
And it could be that Imu isn't involved in Harald's murder whatsoever.
This could have been some 3rd party that we don't know about, not all roads lead to Imu.
Also, narratively, you don't hype up Rocks by sacrificing an Admiral only for it to end up false.