it's a small jab at your expense folks, don't take this too personally. This is a manga about people becoming more and more egotistical, they are not nice, they don't care about each others sensibilities, or yours. And reminder that humanity will always appropriate words to denote that someone is a special kind of dumbass for the express purporses of shitting on each other.
There was once a time they figured they'd call people with disabilities mongoloids! Now that, that was fucked up. It associated a whole etnicity of people with the physical characteristics people with mental disabilities developed. And of course, society seized that. And since the word then got a bad conotation, medical professionals figured they needed a new one.
And the cycle kept going, febble-minded, mentally impaired, mentally damaged, they come up with shit, people seize it to insult each other, and so will it ever be until they pick up an actually hard to remember description for the thing, heck, retarded is not even used anymore because of that exact reason! You don't see a doctor going "Your kid is retarded anymore" because they know it sounds bad.
Don't get all insulted over a word, it just makes y'all look bad.
And if y'all think that the behavior they're showing in the story, or if someone calls out that word in real life on a similar circunstance, is an instance of them trying to insult people with learning disabilities.
Well.... That's pretty retarded....