That doesn't eliminate the possibility of pre-emption though does it? A theoretical version of setTimeout that triggered immediately suspending the active code

That doesn't eliminate the possibility of pre-emption though does it? A theoretical version of setTimeout that triggered immediately suspending the active code and executing the timeout function then resuming (or potentially not). That only requires the interpretor to execute one piece of code at a time. The notions of thread safety would apply to the interpreted code, but not to the interpretor itself.