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.
I found the first 20% of this book a bit tedious as I got used to the style of English, but I'm glad I stuck it out. Eventually it became very natural to read and the beauty of the language is something I'm not sure I've encountered anywhere else.
At least through a contemporary lens I didn't get the impression it was political whatsoever. What it did seem to do was fill in the (many) blanks present in the corresponding biblical narratives.
Using religion as a basis for political ideology is nothing new.
In case anyone is interested, the full text of Paradise Lost with helpful annotations is available online at Dartmouth:
https://milton.host.dartmouth.edu/reading_room/pl/intro/text...