I'm not sure the commenter disagrees with you. If a player wants their character to have a wheelchair, they are simply "saying" that some people have wheel chairs.
I had to go look up diegetic essential ism and basically the idea is that people get too hung up on the literalness of a character in a wheelchair.
What does it say about the universe that elephants can jump ten feet vertically? What does it say about the DM if they rule against this "fact" of DND?