Some editors, like Emacs, make it very easy to work with one huge file, because you can have multiple independent views ("windows" in Emacs parlance) into the same file. It is - or maybe has been - quite common to work like this in LISP communities.

Yes.

As an Emacs programmer, I much prefer working in one 10kloc file than 20 500loc files. The later I find pretty overwhelming.

To bring some balance to the universe - as a vim user, I tend to prefer doing the same.

From a cursory glance, it doesn't look like it's a bunch of files concatenated together.

It's C so it's harder to tell than other languages, since it doesn't have separate namespaces for everything. You can just write a bunch of functions and global variables in separate files and then act like they're all in the same file, and there's really no difference.

You could, but I'm not seeing any code duplication across files so that would mean that he's not included the files that make up the "mega file", which would be rather strange.

Not really that strange. You can have one C file that #includes the separate files that make up the big translation unit. The compiler will give you an error if a declaration is out of order.

Any chance he has a super loooong screen? XD