What is $post->pre_post_content exactly?

There’s no such thing. pre_post_content is a filter that allows you to filter the result of sanitize_post_field() for the post_content field of posts. But the standard WP_Post class, which $post usually is, does not have a pre_post_content field.

If $post on your site has such a field, then it’s likely being added by a plugin, but a web search of $post->pre_post_content turned up nothing, so I’m not sure where you got it from.