It was never said that Harald was Imu strongest weapon at the time .
Also we know that Abyss contract > Deep sea contract>> Domi reversi is not the case in terms of power .
Since normal rocks was beating deep sea contract people like garling and other HK .
So DR rocks could have even possible killed HK .
Harald was the strongest weapon given even base harald is above gorosei like saturn.
You are not getting the point. Base xebec >>> Base garling without imu amp. Hence garling became a laughting stock as even with HK amp he was a fodder.
Your base power matters of course. If xebec had gotten abys contract powers for example , he would be above DR amped version of himself.
For a given same character the hierarchy follows. Unlike garling , base xebec and base harald were shown to be equal. Hence Hk Harald is stronger than DR xebec.