"Whether it runs Windows, Mac OS, Linux, BSD, DOS, OS/2, RISC OS, or any other flavour, or has no hard drive it's still a PC."

I think the the term Personal Computer although it should refer to Apple Macs as well is not used that way. Because the PC is an open design that may be manufactured by any company it is distinct from the Apple Mac that is not an open design. When people talk about the desktop or laptop computer they are referring to machines that probably are running MS Wndows.

Historically we are using a term that used to be "IBM PC" and this became shortened to PC - the key is the common hardware platform. The Apple Mac is not a PC becomes it doesn't have a common hardware platform as do all other personal computers.