Ebert’s isn’t reviewing the book.
Lewis never attempted to downplay the Christian allegory, and Ebert didn't attempt to hide that fact from his readers (even though he was, as you would want to know, only reviewing the movie, and not Lewis' books).
But Ebert has a different standard when the intent of the authors is reversed. Pullman has said repeatedly why he wrote his triology (to "kill God" and destroy Christianity).
If Lewis' Christian faith was pertinent in the LWW review, Pullman's (militant) atheism is pertinent in this review, yet Ebert leaves it totally out.