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.)