Anybody can say anything. They held a trial. Testimonies were given under oath. Other witnesses testified.
You can’t throw out every conviction after-the-fact because somebody says something new. It would be trivial to overturn sentences and lock up the courts for decades.
Don’t be obtuse. Multiple lines of evidence were presented to convince 12 people that he was guilty.
Guess we should just release everybody from prison because we can never know with 100% certainty that anyone ever did anything.