In Skypiea, there was a moment where Enel counted Luffy and Nami as being "out of the competition" despite them being completely fine. In other words, his Haki couldn't detect them anymore once they were eaten by Nola. So, it would be fair to assume that the signature of another living being surrounding you can disrupt Observation Haki. Could it be that if someone among the Giants actually tried to eat a Holy Knight, their connection with Imu would be lost, and they would be left vulnerable?