It seems to me that the biggest problem with that specific situation, assuming the description is indeed correct, isn't that the admins of a forum have the power to edit content on their own site, but rather that if the content has been edited by the admins then this isn't necessarily being disclosed transparently to others, and by extension the original author might still be held responsible for something they did not in fact write.

You might expect that, for legal CYA reasons if nothing else, a forum of Reddit's prominence where controversial subjects are sometimes discussed would at least keep some sort of irrefutable log of any changes made by its admins and clearly mark any content that has been edited by admins to avoid misrepresenting the original author. However, going by various other comments made over the past couple of days, it looks like this might not be the case, which seems dangerous for both Reddit and the people writing the content that gets changed.