Sorry, I misremembered all that. It's hard to find the original information. One of the devs revealed that he was working OpenMCL on a Mac ... but then Reddit was actually running with CMUCL on FreeBSD. (Yes, really!) He couldn't basically even run all of Reddit in the dev environment.
(Regarding "commerical Lisp"; that's what some Lisp people were saying should have been given consideration.)
- They tried using some commercial Lisp on a platform where it was poorly supported, and were struggling with certain stability issues (I don't remember the details).
- A team member came on board who, what do you know, had also authored the Python-based web framework that they were using in the rewrite. Reddit may have been used as the "guinea pig" for improving this web framework.