False cofessions certainly exist, but in every documented case I can find, they have been later recanted. JM has never stated that he was not involved, even maintaining his own guilt after conviction. I don't understand how his low IQ is apparently the cause of his "coerced confession" and yet supporters refuse to believe that his low IQ could also be an explanation for the few discrepancies that are present in his multiple cofessions. For a mentally challenged man, he sure got most of the facts right.
JM effectively recanted when he plead not guilty and refused to testify against his alleged co-perpetrators.
I'm not sure JM got "most" of the facts right, at least not without considerable help from his interrogators. But in the end, it isn't really a question of quantity, but of quality. Whichever figure one accepts for JM's i.q., he knows day from night. As for what he allegedly got "right," despite WMPD testimony to the contrary, there are plenty of reports that the details of the ongoing investigation were widely discussed in the small town. It's hard to say what JM may have heard via gossip.
What "I don't understand" is how people are willing to accept the parts of a confession that support guilt, but simply ignore the parts that indicate the suspect is coughing up fiction. Obviously self-serving statements are one thing (and quite common: "I was there but so-and-so was the mastermind"), but being off on the time of the crime by 12 hours is pretty major in my book.