It's possible that he managed to get Harald mind back for a moment due to his fruit, but the kill was due to his aCOC, we even see Harald commenting on the blood of king flowing through Loki before he pulls his final attack
That means rookie Loki's COC potency > Roger. Imu had no issue with HKs going multiple times after Roger Pirates despite getting taste of Roger's aCOC in God Valley, meaning he believed Roger wouldn't be able to kill HKs, just temporarily stop their regen.