It makes no sense to pick a specific word and draw an arbitrary line. You can be far more offensive without any "slurs" by asking if one's mother was frequently drunk while pregnant.
If what you're saying would be true, then calling someone an idiot would be an insult to all the idiots out there. They are clearly not the target of the insult, the person you say it to is the insulted.
"oh, he hurt my feelings, stop him"
It's fucking moronic for us to have demonised speech like this, we should have held on to the old "sticks and stones" because frankly this Orwellian alternative is bonkers.
We're long past the point where feelings need to actually be hurt to take action.
Half the time it's "oh, he might hurt someone else's feelings, stop him"
Or in short, of course it's an insult to the idiots out there. That's literally what makes idiot an insult.