I haven't read the book yet, but the introduction notes that people with intellectual disability fall short of many of the traditional philosophical criteria for personhood, notably, the ability to reason.
I googled a couple of the authors and came upon this fascinating podcast of a talk by Sophia Isako Wong, an associate professor of philosophy at Long Island University in Brooklyn, New York. Wong's presentation is titled Duties of Justice to Citizens with Cognitive Disabilities and asks: "Do we have different or lesser duties of justice to citizens simply because they are labelled with cognitive disabilities?"
She looks at Harvard philosopher John Rawl's theory of justice which includes the fully-cooperating assumption: "I have assumed...that while citizens do not have equal capacities, they do have, at least to the essential minimum, the moral, intellectual and physical capabilities that enable them to be fully cooperating members of society over a complete life."
Some interpret this passage to mean that people with cognitive disabilities don't count as fully cooperating members of society.
I encourage you to watch Sophia's podcast (I haven't watched the entire presentation). I have taken the book of essays out of Holland Bloorview's library and the content looks fascinating. One of the editors -- Eva Feder Kittay -- is a philosophy professor at Stony Brook University and has an adult daughter with intellectual disability. I think Eva would be a wonderful contributor to BLOOM and hope to seek her out.