You are so bafflingly wrong that I don't even understand how you could ever have gotten there. You're claiming defamation cannot exist, which is obviously falsfalse on its face.
Again; "opinion" is not a legal shield against defamation claims. Once you move past personal preferences like "I have avocado" or "Star Wars is dumb" and make claims about any actual person and any thing(s) that person said or did in actuality, you're no problem longer just expressing opinion, and if you don't have good reasons for thinking that, potentially being defamatory.
Absolutely can and do get sued for defamation if the authors are reckless about accusations, all the time. This is literally the definition of libel.Example that I gave already such as Autobiographical Movies or Books.