Nothing surprising tbh.
GV Roger and Garp is a stronger pair than Shanks in his twenties and Gaban after 10 yearrs of retirement.
On top of that for some reason people completely ignore the fact that Loki was Harald's son.
Sure Loki was stronger than Shanks or Gaban after his power ups ( Ragnir + Elbaph legendary devil fruit). But I'm not sure that Harald would have done what he did against anyone else. He loved his son and was proud of him, relieved that Elbaph was in safe hands with his heir. He literraly let him hit him without any protection to end him.
What Roger, Garp,Shanks or Gaban had difficulties with,was bypassing the CoC coating of Harald and Rocks,inflicting enough damage to overpower the regen.... Loki had a free hit to end things,his father wasn't fighting.