I'm not sure I understand. It's premised on "vim users who can customize vim with C programming", but I've never used C once in my ten years of using vim--customizations are generally done using vim script, which is sort of like working in csh or bash. And, it rarely takes more than a half dozen or so lines to do just about anything I've ever wanted to do (though some complex scripts are much longer, of course).

So, technically this is really the "best explanation of why the creator of this image mistakenly believes Emacs is better than vim".

I can't imagine how one would write something like Slime without really digging into Vim's internals (written in C).

I challenge you to point me to any Vim plug-in (for any language) that provides a REPL like command-line, objection inspection, debugging, etc. without using C to directly mess with Vim's innards.