This is a fairly new “theory”, but was admitted to be being an outlandish bit of fiction soon after the book came out.
"Outlandish" is an exaggeration of what actually happened. The book was preceded by the release of Oliver Stone's JFK and so the focus was still on the movie.
The book got some bad review by unskilled skeptics who chose to believe the Stone narrative rather than the undisputed ballistic evidence gathered by Donahue.
Against the backdrop of Stone’s garish melodrama, the idea that President Kennedy may have actually died as the result of a freak accident seemed by turns ludicrous, inconvenient and banal. Shoved under by a few derisive reviews in mainstream publications like the Washington Post and Time, Mortal Error soon slipped beneath the waves of popular culture and all but vanished from the American consciousness.
One could equate the treatment "Mortal Error" received from a skeptic media to that of President Trump when he first claimed that he was being spied on by the FBI......