Actually this is a question from enthuware's 1Z0-894 set of questions. So I'm guessing the answer they've given is wrong?
Ok, enthuware doesn't make many mistakes, if I just read the API :
true if the context initialization parameter with the given name and value was set successfully on this ServletContext, and false if it was not set because this ServletContext already contains a context initialization parameter with a matching name
It looks like it is still thread safe. Just check the Servlet3.0 specs (I can't get to it from here) and search for Threading Issues to verify whether this is still the case.