Harald already proved his strength earlier in the flashback, actually read it next time. He´s portrayed as Rocks D Xebecs equal, who the government believed could have potentially stopped him. Gaban was borderline in his prime, he clearly looks different then he does as an explicitly old retired character.... and with help, he could do nothing to stop Harald, with Loki explicitly out preforming both of them and amazing Gaban, a character who saw all the strongest in the world before. And are we going to ignore Imu, literally shook at what Loki accomplished, lmfao?
Loki is very much stronger than Roger and Rocks, that´s literally what the flashback was all about. Loki went from looking up to the legends of piracy, only to surpass them and do what they couldn´t.