Maybe Imu is the Six Paths of Pain all over again. Nagato can share his power through the corpses, but he cannot be harmed unless his real body is targeted. Similarly, Imu is immune to anything, as long as he uses others as avatars. The HK and Gorosei can be harmed with good enough ACoC because their real body takes a hit, but Imu in those circumstances cannot, because his real body is not physically present. (Though we saw with Joyboy's Haki how it can even affect Imu from distance, which might also be due to nostalgia).