This is only true for SBCL on Linux, isn't it?
Besides Linux, SBCL supports multithreading on FreeBSD, Mac OS X, and Windows.
Last time I tried SBCL on FreeBSD, the binary package didn't have threads enabled, and when I compiled it from ports, the configuration dialog had me enable the threading manually and said it was "experimental".
SBCL is open source and free and complete and performs incredibly well with multi-threading etc. I wonder how old this article is?