You're right about "PC" as used in the past. The 80s were a time when there were a lot of choices for a home computer -- not just different brands that behaved identically (e.g. dell v. lenovo), but totally different visions about home computers: Atari, TRS-80 (I had a 16k CoCo), Commodore, Apple, IBM, TI-99/4A, Coleco Adam (my cousin had one of these), and I'm probably missing a dozen other manufacturers or more.
These were all distinctive systems and back then, "PC" really did mean an IBM, and then later "PC Clone/Compatible" came into vogue for non-IBM computers that functioned like an IBM. Nobody called a C64 a "PC Clone" exactly because people expected "PC" to mean something that worked like an IBM PC.
Today though, it seems the distinction has sort of faded away. IBM doesn't even make PCs anymore, even the clone market is highly consolidated, and the hardware is basically intercompatable no matter where you get it -- you can install Windows on your Apple if you want and people have been doing hackintoshes for quite a while (running OSX on non-Apple hardware). Somewhere along the way in all of this, PC seemed to become a rather generic term -- I can't pinpoint when -- maybe around the time Apple went Intel.
It's a shame in a lot of ways that there is so little computer variety anymore -- maybe someday we'll suffer a "great computer famine" due to the intensive monocropping consumer computer gear has experienced.