I can't believe they didn't change the backstory for him so that he always had a dark skin tone when they had the chance. If the anime gets a season 3, it'll probably make it in to that too now...
The in-universe explanation is that Saitama had such a low potential that doing the most basic of exercise routines pushed him over his limit (essentially the baseline wherein you become unbeatable busted trash) into OP territory, and other heroes had a higher potential baseline and as such didn't break their limits. The actual explanations is that it's a gag manga.
I don't think that's quite it either. The limiter thing is a misdirection at best, mostly.
OPM basically runs off how strong you believe you are is how strong you can get. Saitama strongly believed in his goal that he would be the strongest hero, and that his training regimen is what you needed to do to become the strongest hero, so he ended up there. He can't get out of there because he still believes he's completely unbeatable.
This applies to everyone, for instance, why Mumen rider is stuck at the top of C class. He doesn't believe he could be any better than a C rank hero, and so that's where he plateaued. Boros also believed he was unbeatable except for the challenge he was told he would find on Earth, which when faced with saitama who believes he can never have a challenge, he didn't stand a chance.
It's also why in the chapter being discussed here, it explicitly says that Darkshines goal wasn't to be an unbeatable hero, and in a prior one, Darkshine only started to lose because doubt entered his mind that could win.
So when they say stuff about the limiter, it's actually more about what the person believes they can do rather than anything else.
