Now we know that one of Loki's defining personality traits is denial. Whenever he loses, he refuses to acknowledge it.
When Imu completely destroys him and leaves him on the verge of death, Jinbe says he was defeated, and Loki responds, "Who did you say was defeated?" completely rejecting the reality of what happened.
The same pattern shows up in his past. After being defeated by pre-Yonko Shanks and ending up chained to a tree, instead of admitting he lost, Loki once again goes into denial and calls Shanks a coward.
This is a consistent part of his personality. Loki would rather reinterpret what happened than admit he was defeated. His instinctive response to losing is to deny it or discredit the person who beat him in order to protect his pride.