> For a fictional portrait to be actionable, it must be so accurate that a reader of the book would have no problem linking the two

Sorry this argument makes no sense. If I (or any average reader) read a passage dissing a public figure (not me), which describes them with a small penis, I wouldn't consider the description as not fitting - I have no way of telling how big their penis is.

If the public person in question came forward, and read the passage, he could successfully argue, that readers of the book would have no information about the size of his package, and thus that would be irrelevant to the argument. So him suing the author based on this would not mean he admits his dong is small.

Maybe it doesn't have to be a literal small penis? Maybe it could be describing the person in such a way that it's clear they are the one being described, but then including something about the character that they wouldn't want to risk people thinking is true.