The book being reviewed by TFA is not itself a review of Paradise Lost, but a study of how people have interpreted Paradise Lost since it was published in 1667. TFA says 'The biggest story that Reade is telling is that of slavery.'

YMMV, but I don't think that this was Milton's main message - IIRC from reading PL years ago at school, the main story was Satan's rebellion against God. Satan was presented as a sort of heroic anti-hero, who has some great lines, although he eventually (of course) loses. So, I guess I'm saying please don't let this review of a book that highlights lots of peoples' reactions to PL influence your judgement of the underlying source text.