You said a flaw is still a disability even when everyone has it.
Where did I wrote that?! I wrote that the hypothetical situation where everyone has a broken leg and therefore then it would be considered normal, doesn't invalidate that a broken leg is a handicap in our (non-hypothetical) real life.
Also, being double jointed is not considered a disability.
And further, the word is clearly defined (this is translated from my language to English):
Physical, mental, intellectual or sensory impairments which prevent a person from participating equally in society for longer than six months.
And what we categorise as a disability is grounded on the definition above. Since autism is categorized as a disability, it wouldn't make sense to diagnose someone with autism if the above is not true.
I don't see how this can not make sense. It seems so obvious that you do not have a disability when nothing is disabling you. When someone says "I have disability X but it's not disabling" then congratulations, you are cured.