Imu's Haki wasn't anything abnormal; we can see that other giants besides Brogy and Dorry were fine. Joy Boy's, Shanks's, Kaido's, and Luffy's Haki are still the strongest.
Then, how did Imu get caught? Literally, there were 50 giants running towards him — the noise alone should have been enough for him to notice, yet a giant the size of a mountain managed to catch him. If it had been at least a top-tier like Luffy in his giant form, catching Imu just like he easily caught Kizaru, it would be justifiable. But a trash giant doing that is embarrassing. I hope he shows Future Sight, like Kaido also has