What are you talking about? The piece you linked doesn't include the percentage of post-mortem exonerations at all. It doesn't even deal with them. It just says that most prisoners have been there for long periods of time and any exonerations that do occur happen well after a decade has passed, which has led to longer and longer death-row sentences, a sign that the death-row system in the U.S., to the degree that it is used at all, is fundamentally broken, as it has essentially become life in prison, not the actual punishment handed down by the court.

The U.S. has undoubtedly executed innocent people, particularly before DNA evidence was prevelant, there was a case in 2009 where the Innocence Project of Texas proved using DNA evidence that a man who was falsely convicted of rape was innocent post-mortem.

However, how many, and exactly what percentage is will probably never be known, as the bar for an appeal and exoneration, particularly post-mortem, is high (requiring new evidence to be presented). This is also compounded by the general lack of interest and funding for pursuing these cases. With the limited resources of innocence projects and the fact that you can't put the cat back in the bag, most projects focus the majority of their efforts on the living.