I heavily doubt that Imu literally went 1 vs the world.
First of all, the mural doesn’t even show that, there’s a boat of allies at Imu’s feet.
Secondly, there are the other nineteen kings. It’s pretty clear that the Celestial Dragons are strong, and that’s going to be at least eighteen (Lili’s allegience uncertain) decently strong people supporting Imu. We’re talking about the ancestors of Shanks, Doflamingo, the five Gorosei. Anyone really suggesting all these guys just stood back and watched Imu fight?
Finally, there’s the idea that the God of Forest sent out demons, plural. Suggests to me that there’s been other iterations of Imu leading the Gorosei- was Egghead not basically Imu sending out demons?