Great article. It’s very easy to make this mistake. It takes active thinking to train yourself not to.
If you are disabled or have a family member with a disability, it also helps to “train” others with your response when they do it. For instance, I have a 19-year-old with Down syndrome. Sometimes people see her and assume she can’t understand speech, so they ask me a question that should be directed to her. In that case, I ask her the question instead of answering for her. That politely makes the point that she can speak for herself.
I also try in turn to talk to people with disabilities directly, even when I’m pretty sure they can’t answer back. That way, I’m acknowledging they are there and involved — and if I’m wrong, they can speak for theirself.