Most of what you said is exactly my point. It’s true the word Linux, used properly, refers to a kernel and not an operating system. But that’s not the way the word is used in practice, and it is not what OP meant when they used it. They meant " an OS with the Linux kernel and GNU userspace utilities." When the word Linux is used that way, Android is not Linux.
If you can give a reference to any such book, I’d be very interested to see it.