The whole reason the phrase "IBM PC" existed was to differentiate it from the other PCs that already existed. "IBM" was the adjective. "PC" was the noun.
Because of its success in offices, "IBM PC" became just "PC" the same way other words like "omnibus" became just "bus" because it's simpler to say.
Since then, you might refer to any of a variety of machines as "personal computers", but "PCs" only meant "IBM PCs" (or later "IBM PC-compatible machines"). In other words, I would argue that the term "PC" derives specifically from the "IBM Personal Computer", and not generically from "personal computer".
Source: I was there. :-) I haven't done the research, but I bet if you did search through Byte and similar magazines of the time, you'd find plenty of supporting evidence. (I do have a memo cube from the early '80s with the slogan "Apple II -- The Personal Computer", but I suspect that was Apple Marketing trying to fight an ultimately losing battle.)
I do disagree with the suggestion that the initialism "PC" was understood to mean "personal computer" in general before the release of the IBM PC. If that's overly pedantic, well, I'm a computer nerd; what can I say...
https://books.archivelab.org/dateviz/?q=home+computer
https://books.archivelab.org/dateviz/?q=personal+computer
Personal computer was used a lot more when books were eventually written about the era.
"Home computer" was things like the VIC-20 and the TI-99 4/A. "Personal Computer" was machines that you had in your home or office that you didn't have to share, or timeshare with someone else. Think Cromemco, Kaypro, PET, and SuperBrain.
But in this case it was used as "desktop computer", which sounded strange to me as I consider laptops, however portable, to be personal computers.
You don't think Apple's marketing is behind the aforementioned recent trend; do you?
You're misremembering history. It eventually evolved into that, yes.
Apple didn't really buy into that until the "I'm a Mac/I'm a PC" ads.