The article targets MRI (Matzes Ruby Interpreter). It does not execute threads concurrently. Probably for the same reason that Python imposes limits on threads - to stay single threaded when calling into C libraries, most of which were written before threads were important enough to think about them

While that's true for the running program, it doesn't stop the JIT engine running a thread to compile code as the parent comment suggested. It's not running the ruby code.