I did learn to write academic essays early, my writing was evaluated and graded based on my ability, and there were other peers at or near my intellectual level in my classes with whom I got to converse. I also got a lot out of conversing with all of my peers.

Because so many schools, like, you, don't believe all of the evidence showing that differentiated instruction works, most of them are indeed "exceptionally bad at it."

No one is arguing that "significantly intellectually delayed" kids shouldn't receive special education services. Those children are a small minority in pretty much any mainstream school, though.