Autistic character in book

I'm reading the latest Stephen King novel "End of Watch" - the final book in the Mr Mercedes trilogy. There's a character in it called Holly who's obviously Autistic, but this isn't acknowledged and most people except for the main characters - who all love her obviously - can't understand her behaviour and treat her unkindly. Has anyone else read this book? I can't work out if the author is trying to get the reader to discover that this character is Autistic, and so give them an idea of what it's like to be "different"? Why would an author write an autistic character into a story and then not make it clear that is what they are?