As sibling said, reddit was CL before it was Python. But more importantly, reddit predates Arc by more than two years. :)
And the main reason reddit was rewritten was library support. Doing simple things in CL was hard because you had to write it from scratch, but doing those things in Python was a simple library install.
It's easy to roll anything in CL if you know what you're doing, but making it talk wsgi or generate pngs isn't trivial, no matter what language you're writing it in.
I think so? I honestly don't know, I never really worked with the CL codebase, I just used it for reference sometimes. But even if it did, I'm not sure how much velocity is gained by using CL vs. Python, especially when making a web app, where there are so many examples, libraries, and frameworks out there to help in Python.
While I have admittedly limited experience with it, CFFI worked awesome for the couple things I tried to do with it. I don't have any CL code in production yet, but enjoy using it when I'm doing some recreational programming, or looking for something that's easy to embed in a C program (Lua's nice, but it's trivial to take the ECL Common Lisp implementation and hook up emacs-slime to it, even when it's embedded in a C program)
Lisp shines for codegen and creating DSLs and new control structures, but if you're just writing out tedious imperative code, it isn't really better than Python. Its lack of popularity means it can't offer as many libraries waiting to be reused.
And the main reason reddit was rewritten was library support. Doing simple things in CL was hard because you had to write it from scratch, but doing those things in Python was a simple library install.