Things were more complicated than that with Reddit from what I've read (and from a now defunct blog post they wrote not to mention various talks and interviews from devs who were there at the time).
Their devs were using Macs in 2005 which ran on PowerPC. Their servers were x86, but running FreeBSD (honestly, that was a tall ask for most languages in 2005). They had an issue finding threading libraries that worked on that OSX/PPC and FBSD/x86 combo. They further complained that there weren't tons of libraries available for use either. Finally, they also made some bad architecture decisions unrelated to Lisp.
The switch is still a weird one if you move aside from the new team not knowing or wanting to learn Lisp. Python isn't threaded at all, so they could have stuck with non-threaded CL and still have had 100x faster code. Likewise, they wound up rewriting all the python libraries they were using because those libraries turned out to have tons of issues too.
Their devs were using Macs in 2005 which ran on PowerPC. Their servers were x86, but running FreeBSD (honestly, that was a tall ask for most languages in 2005). They had an issue finding threading libraries that worked on that OSX/PPC and FBSD/x86 combo. They further complained that there weren't tons of libraries available for use either. Finally, they also made some bad architecture decisions unrelated to Lisp.
The switch is still a weird one if you move aside from the new team not knowing or wanting to learn Lisp. Python isn't threaded at all, so they could have stuck with non-threaded CL and still have had 100x faster code. Likewise, they wound up rewriting all the python libraries they were using because those libraries turned out to have tons of issues too.