You are assuming edits overwrite existing content. Instead of overwriting, they could just store the edited post as a new entry in the database with a higher version number. Then, you only show the latest version of each post to the end users while keeping the older versions available die Reddit’s own use.
In fact, it is extremely likely they do this. It is basically a necessity if you want to be able to properly moderate a site like Reddit. Otherwise you could simply post spam or unsavory content, and then overwrite it with something benign an hour or so later, before there were enough reports and a moderator would have gotten a chance to review it.