The way I look at it, "Mac OS" is short for the "Macintosh Operating System" which it is. When I say "Mac OS" I'm usually talking about the general eco-system for the Mac and the OS itself.
Now I do have some older PPC machines and some 68k machines, most of which are running pre-OS 8 for their software. Usually, I refer to anything 7 or earlier (7.6 included) as "System Software" or "System". I also sometimes call OS 8.0 System 8, but only when I have been talking about earlier versions of the SS.
When I started using Mac OS X, it was only obvious to me that it would be "Mac OS Ten" since it was called "Mac OS Nine" before. When it comes up in conversation, per-say someone asks me what OS I'm running, I respond "Mac OS Ten version 10.9.2" because nowadays the official OS name does have the X in it. The "X" is the roman numeral, you wouldn't look at VIIII and say "v, i, i, i, i". You would say "nine" right?
Also now that I think about, Steve and Tim have always called it "Mac OS Ten".
So, no "Mac OS" is not an incorrect term. Just because Apple dropped the Mac from the OS's name, doesn't mean its an incorrect term. People have said Mac OS since the beginning so they didn't have to sit there and say the whole thing, "Macintosh Operating System" even back into the System Software era.