The context of feeling that haki was him getting ready to battle it ergo it was a powerlevel thing.
No, that wasn't the context, hahahaha!!! Luffy never said Imu's haki was the strongest he'd ever felt, he didn't use the word strongest, he could've been talking about impurity, about sensing something abnormal in that haki, something different... If it were the strongest haki Luffy witnessed in his whole journey, he wouldn't be saying he could beat Imu after eating, "I feel like I won't lose to anyone the way I am now," right after seeing how strong that haki was? Hmm weird