Yes and no, of course yes not everyone will understand everything. Similarly to you I have a degree in Social Science and a MA in Disability Studies and my understanding of advance physics would be about the same as yours whether I read it or someone read it to me. However, over my years as a TA with 16-18s and 18 to 30,40,50..., I could give many examples of students who could read something and not get it, but have it read to them and get it.
In my original example, yes of course there are people who wouldn't get the work place structure document whether it was read to them or they read it, but there are also people who wouldn't get it reading it themselves, but would get it being read to them. It IS two skills, but in the real world you need BOTH skills in order to gain information from texts, which is normally the point of reading (whether you are gaining that information for pleasure as in making sense of a novel, or to gain knowledge).
In OPs original post she was arguing her son shouldn't be given easy reading books just because he can't explain what he is reading, but later in life he will probably NEED to be able to read TO understand what he is reading!
Even if he is an excellent decoder for his age, if he goes through his school years being able to decode but not gain meaning, then the minute he needs to understand a text to use in coursework or understand the instructions on homework task, he will struggle to complete the task he is doing just as much as if he couldn't read the instructions. At work, if he can read instructions for machinery or how to give a medication safely, but not understand what they mean unless someone explains them orally, he will not be able to complete these tasks appropriately and safely without help any better than if he'd needed someone to read the instructions to him!