I think this is a great powerscaling chapter. Oda just cemented what the most sane of us already know.
Imu >>> Xebec >= Harald >>> Admirals.
Xebec directly admits he can't do anything to Imu even if he wanted to. He made sure to let Imu know that he'll be back, though. Immediately got into recruiting the strongest people into his crew (likely by defeating them in a classic Davy Back fight) and tried to bring Harald, all so that he could have a shot at taking down Imu and conquering the world.
Let's not even elaborate much about the admiral he killed because Oda himself treated it as nothing worthy of highlighting it further; no name, no clash, whatsoever. He just used it as a hypetool that cements Xebec's power is at a level where he can kill admirals as a side quest.