I think the reason people jumped to the defense of trans-women in another thread and might not do the same for disabled people is that being trans is a bit more specific, defined, and trans people are a bit more unanimous in not wanting to be called "not real".
Disabled people I know are all over the place. For instance a relative of mine mentions pretty much daily how lucky I am not to have her disabilities and how she wishes she didn't have them (daily might be exaggerated). One of my close friends, also in a wheelchair, uses his disability to tell everyone that he's the living proof God can't exist (although he never straight out complains about it).
My relative takes offense when it's implied that she's disabled, but does say she is whenever it's practical (I'm not kidding here. She'll ask someone to go get her something from the fridge because she can't do it, and five minutes later goes out to buy cigarettes).
My friend uses the word crippled and says that the word "handicapable" is an insult to anyone's intelligence.
I don't think either of them would have found the sentence in question ("I'm lucky not to be disabled") to be insulting to them. I think both of them would have reacted with "Thanks for understanding, most people can be so cruel" and "No kidding, next thing you'll tell me the sky is blue".
So it's hard to jump to your defense when we don't know that you need defending.
And there are so many different things we call disabilities, with different causes and different symptoms. So it adds to the complexity of the issue. How do we know the way you want to be treated? Well, we know because you tell us. But in that other thread, you told us by insulting other people, not by politely stating something we had no way of guessing. So that wasn't very helpful.