Most people mean "Windows machine" when they say "PC" though. Since the way people use a word define the meaning of the word more than even a dictionary definition, PC does in fact mean Windows machine.
In the "I'm a Mac, I'm a PC" context there is no confusion what PC means. In Hacker News context I can understand how the term PC might be ambiguous. I am really arguing that at this point though even hackers are calling Windows machines PC's. Although I have always wondered why everyone refers to them as PC's I have never found myself being confused when I hear it nor have I ever been misunderstood when using the term.