Where did you get that from? Lol. I have two autistic children, one of whom limped through high school, got an EHCP, and never managed uni. I also work in SEND.
If a child's needs can really only be met via small class sizes and an independent school offer then those parents should apply for an EHCP to state as much.
Oftentimes moving a child with SEND out of mainstream just masks the problem for a while. It leaves children without the legal send protection they need or, potentially , their needs being correctly identified. Then they move back into the state at sixth form or uni and are plunged into instant crisis because they would and should have had a support package years earlier had they remained in m/s state.
I fully understand why parents might decide that an independent secondary school feels like a safer option for their child. That is still a choice. There are plenty of children who are autistic or ADHD in mainstream whose parents didn't have the financial option to make that choice.
I don't agree that children whose send is solved by going to a smaller school, bright enough to get decent A levels at that school and who are independent enough to then head off to uni are such a distinct and vulnerable group and they certainly aren't the majority of children in independent schools.