The author is referring to the GIL, I’m pretty sure. Both Python and Ruby have it. I guess, they should’ve written “only Clojure supports true multithreading”, but that you’re able to spawn multiple processes is kind of a given (and has quite some disadvantages for certain workloads).

That Python has experimental no-GIL builds is good, but I wouldn’t seriously consider using that until they’ve had it in a stable release and thrown some bug fixes onto it. Well, and even then, there might not be much point in trying to use it, until libraries have adopted it…