"Have the words 'accessible' and 'lucid' been redefined? "
I'm afraid they do seem to have been redefined nowadays, as 'dumbed down', which this book is not. Perhaps some readers will find that disappointing. The book is accessible to anyone willing to give sufficient thought to subtle, and often novel, concepts.
The sentence you quote from page 3 is incomprehensible taken out of context. In context, it is referencing a hypothetical religious doctrine, outlined earlier, to help illustrate characteristic differences between religion and philosophy. All this is perfectly lucid, at least to a reader who is willing to make a little effort (without which, nothing worth learning is ever properly learned). More opaque is how you could find a book disappointing 'overall', when you left it 'largely unread'.
"...beginning a sentence with the words "Less abstractedly" does not render what follows less abstract. "
True, which is why the sentence begins with the words "Less abstractly". Perhaps you should have another attempt at reading the book, this time less abstractedly.