What you’re referring to is the historical use of a term compared to its contemporary or colloquial meanings. “Idiot,” was indeed used in the past to refer to people with disabilities or mental illnesses, that is very much not the dominant understanding of its meaning today. “Idiot,” today has the meaning of poor intelligence or foolishness, but is not associated with conditions that people imagine result in those poor qualities.

Something like the “r” word is very much associated with mental, genetic, and learning disabilities that are, by its very use, argued to be the cause of the victim’s perceived poor intelligence. They’re calling the person that word regardless of whether they think they have any disorders because they associate those disorders with poor intelligence and also poor social value. It being an insult is dependent on the devaluing of disabled people.

“Idiot,” or “stupid,” do not carry those connotations in culture, they’re only insults referring to the intelligence of the individual(s) they are directed at.

With all that being said, it’s often seen as low-class or immature to resort to these insults at all. You should never use the “r” word as it is a slur directed at an extremely vulnerable group of people and its use reinforces their devalued status.