Because that’s how I used it on the playground when I was a child.

I feel like there is a concession here?

I asserted another person was less due to an intellectual disability, normalizing that it’s OK to insult people using a term that once upon a time neutrally described the intellectually disabled.

This is explicitly what they're saying it doesn't mean. They're not referring to literal mentally handicapped people. They're using a casual if aggressive put-down towards people who demonstrate profound anti-intellectualism in a way those people would understand as a means to viscerally and painfully insult them.

And they're doing so in a way that doesn't mentally police their own language. Self policing perpetually requires cognitive effort and second guessing. It puts you culturally on the backfoot. If you want to fight rightwing demons, you have to go for the jugular on a pure red boiling instinctual level, or you lose. You have to fight so hard that there is collateral damage, otherwise the average person will simply see you as inauthentic and overly clean and calculated.

Further, ableism and other particular 'isms' like racism and sexism are not directly comparable. Talk about bad faith. Ableism based insults directly reference material capabilities in the person you are insulting, definitionally. Racism and Sexism are bad because they prescriptively assign people to irrational and prejudiced stereotypes about their weakness or unworthiness. Ableism just directly references perceived weakness. This is still rude and mean spirited, and thus can easily be bad. But it is simply not the same.

I am autistic. When people insult rightwinger's social behavior by calling them a cringey autist (and yes, I've seen this) it does sting. It does bring about some bitterness because I did not choose to be this way. But I get it... I understand. If it hurts the rightwinger, its probably worth it. And I've even engaged in the same seeming self loathing indulgence to get at people who need to be brought down a peg.

There is no moral difference between calling someone "stupid" and using the R word, except in the impact of the term. And the modern rightwinger gives absolutely no shits if you call them an "asshole". In fact, many of them like that, they'll even embrace their lack of morals with pride.

And then I learned that it isn’t OK to punch down, that the people used as an insult were as human and deserving of respect as myself.

Correction: Its harmful to punch down. Its more harmful to refuse to punch down as some kind of holy rule if it weakens a righteous cause to more broadly protect the weak. By destroying a rightwinger's pride through such humiliation of their material ability you are helping the weak, even if through "splash damage" you also hurt their feelings.

That said: I don't even think any of my politically tactical justifications here are wholly needed. Sometimes you want to hurt who you rightfully hate and are willing to damage yourself and others to do it. Rational ethics do not always come into the picture when you are facing the end of the world. Its just eye rolling to expect clean language as we sink into the fires of hell.

Sometimes you want to emotionally unburden yourself and metaphorically just rip a pigs head off in the mud because the world has shit on you when you did not deserve it. My visceral hatred of the average dipshit rightwinger is so intense as to nearly be the reason I've not off'd myself. I'm almost surviving off spite right now.