That's part of the challenge since not everyone will use the same terms for the same things. I find the "Player Character" as companions tends to be a perspective from more older school types of players, possibly because older games had players create full parties, but that's just my own personal observation so maybe that assumption of mine is wrong.
I can certainly understand your position (it's actually in alignment with mine), but misunderstandings happen when people see things differently.
I used to play old games and if I controlled characters that *I* had made, I guess those were PCs to me but in newer games, I've always felt that the character through which a gamer affects the world and whose agency is basically the gamer's was the PC. We don't make the others, though we could control them for a while so to me, they're not The player's character, rather than the player-temporarily-as-Isabela... But, as you said, it's all subjective. Such variations in personal interpretations opens up many chances for their to be a miscommunication somewhere. Add vague wording to that mix and you get a mess, most of the time.