The book is not a work of scholarship. Rather it is like one student helping another as opposed to getting help from a professor. Many times that works better for learning.
The Psalms are poetry, and with that goes all the licenses that go with poetry. They were meant to be sung.Their chief formal characteristic is parallelism—a phrase repeated with slightly different words.
He handles the the characteristics of the Psalms he finds most repellent. That is akin to eating the nasty stuff on your plate before the good stuff.
This is not written to convince people to become Christian. “A man can’t be always be defending the truth; there must be a time to feed on it."