Well, I would certainly agree that not every picture that happens to have a person in it is a portrait of that person. Like when I was in Sanssouci Park in Potsdam, the place was teeming with tourists. I had persons in ever so many of my frames that I did not even WANT in them. So clearly my photos of those persons are no portraits. In this, I am sure, I am with everyone here on the forum! Everyone.
I also (still remembering our visit to that park) took a photo of a young Italian tourist taking a photo of his posing girl-friend who was sitting on the rim of a bowl shaped pool with fountain. MY photo of the two - though meaning to be OF THE TWO - certainly never is a PORTRAIT of the two. It is just a photo of the two, of that scene, of him taking her photo.
But could his photo (that I never saw, of course, since I don't know those people at all) have become a portrait? What do you think? Or was it only a nice snap of her posing for him. Because all they thought about was the surroundings but did not spend any thought on special lighting (other than the big spotlight up there in the sky) or any other things?
What is it that you think is needed to make the photo of someone's face (and shoulders, maybe) or of someone sitting on a chair in a stream of spotlight, or of someone captured at work in his daily (and life-determining) surroudings a portrait and not just the photo of said person?