>>280691098
it was a losing situation for both of them. Would Rocks be able to kill Imu and Gunko? Maybe. But most definitely not as easy as he stomped over that admiral, leading to him having to face the Gorosei, holy knights and other admirals while trying to ambush Imu.
Would Imu be able to kill Rocks? Probably. But that would very likely lead to their existence getting revealed.
So why would both of them even risk it? Rocks was there to make an empty threat. He needed the backup to bail him out afterwards and both knew.
More interesting is the why. Why did Rocks know that Imu existed. Why did he invade Mary Geoise all on his own to tell them that he knows. Why was he strong enough to do that, while being a complete nobody.