If, to you, saying chud is okay since you don't mean it in the '=ugly' way, why is the r-slur unnacceptable, if it originated from an equally unoffensive definition

This is where the breakdown is happening. Chud's origin is not "chud means ugly person, so when I call someone a chud, it is identifying ugly as bad." "Chuds" were monsters in an '80s movie who lived underground and ate people and people started calling people "chuds" from that movie and they might have used it to insult all sorts of people. When Homer Simpson says "and that's when the Chuds came at me", he isn't talking about people with disabilities. He is talking about the monsters from the movie, which was much more in the zeitgeist in the '90s when that episode came out. And given the social impact the Simpsons had, my guess is that chud's prominence in the lexicon is largely a result of the Simpsons using it, just like "meh" and "yoink."

You can be ableist using non-ableist terms. Some of the examples are elsewhere in this thread where people say people treat them badly, and hurl insults at them, because they do not process social queues in normative ways. So if I call an autistic person an asshole because they don't pick up on my sarcasm, maybe I am being ableist because they might not be able to identify sarcasm and irony very well. They have no malice, it's just how their brain works. So you can be ableist towards people using non-ableist slurs. In this example, "asshole" does not become an ableist slur whenever it is used just because some asshole used it in an ableist way. This is like chud=ugly. Some people use chud to call people ugly, which is mean. But if I call [pick your favorite billionaire] a chud for trying to destroy society, it is not ableist.

The r-slur's origins are arcane medical terminology used to describe people with a wide range of developmental disabilities. When you call someone the r-slur, you are invoking outdated medical terms and then identifying the subject as "bad" because they resemble people with developmental disabilities. That's what makes it inherently ableist. The implicit assumption that the type of person that word was associated with is inherently bad and that being labelled as developmentally disabled is an insult is ableist. Just like calling someone the f-slur to mean "bad" is inherently homophobic.