the euphemism treadmill is a description of linguistic patterns, not a get out of rule free card
Using ableist language as insults is always bad, even if the words seem innocuous. Some may reference the "euphemism treadmill" to try to justify their behavior, but it's crucial to understand that the treadmill is merely a linguistic observation. It does not exist to normalize ableist behavior.
Anything is ableist if that's the intention is to be ableist. "Cognitive impairment" or "Brain damage" are clinical terms, but if one decides they can also be ableist insults.