A similar statistic applies to many disabilities. If you give people a few years of time, after they acquired a disability, studies show their happiness isn't substantially different from a person without a disability. Grief comes from lack of accessibility and society failing to support us. But not from the actual situation. I have to explain this every other week, to a non-disabled person that tries to tell me how bad my life must be. Its a well known phenomenon. And a total break of boundaries. Imagine someone walking up to a woman and tellign her they are sorry for her being born as a woman? Not imaginable. But happens with people with disabilities all the time.
This sounds a bit like you dislike the lack of understanding, which itself is based on lack of understanding.
On the other hand I agree that commenting on ones disability is a break of boundaries in most contexts.
One should quite often avoid to comment on traits in general that are irrelevant for the context or the conversation.
It is not about understanding, it is about not even trying to and just telling a complete stranger how you think they have it bad. That is, as you identified, a break of boundaries. And, frankly, if someone breaks into your house, you are not obligated to "understand" them. You just know they are shitty humans.
