It says a lot about the quality of that lisp code and the clarity it enables that a rewrite could be done effectively over a weekend. Normally the "Big Rewrite" is a story of total failure.
Even if they decided not to go with lisp long term this story is definitely a good advertisement for prototyping in lisp and sticking with it until there is an actual reason to change.
As a lisper, I’ve gone through the code of the original site ( https://github.com/reddit-archive/reddit1.0 ) and it’s not surprising to me that they had to rewrite it: it does a bunch of interesting environment-dependent things during compilation and loading that make it really hard to even attempt to run.
I’m also fairly certain that rewriting in lisp would have been nearly as successful: the benefit came more from learning about the problem space than from switching languages.