I'd argue that "PC" has two meanings then. I'm going for "non-Mac PC", since that's what the title implies by separating Macs from the rest of PCs, and because I don't feel sensitive to exclude a huge set of possibilities.
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.