Firstly, these comments don’t prove that Shanks’ Conqueror’s Haki is on the same level as Joyboy’s. Shanks’ CoC knocked out his comrades, whereas Joyboy’s CoC didn’t affect his allies.
Secondly, Imu had a mental breakdown, similar to how Kaido did when he remembered Oden yet Kaido is stronger than Oden.
Simply put, Imu and Joyboy should be close in strength—there’s no way Imu completely overwhelmed Joyboy.
Imu was affected because the Elders were using her power, but not to the extent that she ended up in a state like Green Bull.
Green Bull was in a far worse condition from Haki unleashed miles away, whereas Joyboy’s Haki was used just a few meters away from Imu’s power-enhanced soldiers.