Who debunked Posner’s book? I thought it was dadgum convincing.
Nobody did. The FR claimant to the 'debunking' has made a gratuitous statement with no facts to back it up; take as being worth as much as "I read in a book somewhere," or "I once heard..." Posner's book hasn't been debunked by anyone.
However, if you're interested in a shorter, clearer, and in many ways more convincing read that you can get cheap and is the source of a lot of Posner's ideas, take a look at this little ditty. Conspiracy of One: The Definitive Book on the Kennedy Assassination (Paperback) by Jim Moore.
A gem.
With just one slight change to the chronology--supported by eyewitnesses who testified to the Warren Commission--Moore effectively demolishes all the "magic bullet," "forward recoil/backward recoil," and "Mannlicher clumsiness" objections raised by conspiracy kooks over the years. I wish Posner had given him more credit. This book shouldn't have been ignored.