I'm 99% sure threads will run on all cores. I think the author has confused multithreading with hyperthreading.
See also http://www.dabeaz.com/python/UnderstandingGIL.pdf, which explains that
• Python threads are real system threads
• POSIX threads (pthreads)
• Windows threads
• Fully managed by the host operating system
• Represent threaded execution of the Python interpreter process (written in C)I can't imagine it's being upvoted.