https://8ch.pl/pol/res/26296.html#26296 <- the actual post in question
https://archive.is/jxcfl <-- archive of actual post
https://www.reddit.com/r/DNCleaks/comments/5cljkp/work_in_progress_wikileaks_the_missinglosthidden/ <-- old work on this
https://archive.is/snd84 <-- archive of old work
as summarised by /u/rush22 in the original post

The guy on 8ch was saying that when you search the podesta emails with nothing in as a search term there's 58660 results. But he noticed the id of the e-mails goes up higher than 58660. For example this email: https://wikileaks.org/podesta-emails/emailid/59229

So he wrote a script to type in the url to download the email source, and then he downloaded from 1 to the highest number, 59258 and he did in fact get 59258 files. This is higher than the search result. He found some of them are duplicate files. But it is still higher than the search result. So that means according to the number of files that search returns, there's an "extra" 439 files that aren't searchable. Or 58660 is some sort of technical limitation of the search but that would be weird.

One thing he didn't question is why the directory in the image above only has 57153 files, but that's something else I suppose.