If they all are truly immortal and lived a couple hundred years, it'd be mind boggling as to how they'd be weaker, considering how much time they've had to grow. Look at Saturn. It seems his body just exudes Advances Conquerors Haki. That's some next level shit.
Starting with the end of it, I don't think Saturn is emitting Coc.
Firstly, no one has ever addressed the CoC black lighting, and on the contrary this was the first thing that happened with Saturn. However, this is primarily because no one, ever and in any situation, reacted to it. Not even the fodder marines fainted, not even kid, salve Kuma. Literally no one and no one has ever addressed it as CoC, mugi in primis.
So I think that effect is just connected to his power.
The only way I could have seen them being stronger than the strongest in the world is if they were from the void century, but we know they aren't.
All in all, WB, Kaiso, Mihawk, Akainu and so on have far more qualifications than them. But we'll see.