my 2 cents is that you're discussing the wrong things here, namely particular techniques and superficial stylistic things, versus more importantly, the intent of the artist.
I think that the history of photography has built into it a number of certain "assumptions" about the representation of reality. Relative to other mediums, most certainly painting, photography is fairly "biased" more towards representational, documentary style and more "objective" portrayals of reality, as opposed to other forms more inherently abstract, subjective, etc. And, ironically, while the history of black and white photography is most certainly stylized and not truly "realistic" -it still has a very predictable and clear "expectation" of what that black and white "world" should look like.
Conversely, now take a look at someone like the great Mario Giacomelli, his stark black and white landscapes. They look nothing at all like any other photographs at all, an "anti-Ansel Adams" vision of nature if you will. They look nothing like reality; Giacomelli was famous for saying that he didnt record reality, but instead "destroyed" it. He photographed HIS reality, not anyone else's. To me, he is the ultimate painter "of" photographs; his photos feel more like paintings than perhaps any other photographer as a result. In this way, he probably has more in common with someone like say Jackson Pollock, who proclaimed that he didnt depict nature, but literally said, "I am nature." Once again, polar opposite from Ansel, who is way more in line with the "traditional" approach of photography visa vi nature. So to me, a photograph starts to become "painterly" when the photographer's INTENT is to portray more of his own reality, or to abstract reality, or express himself differently from the more "conventional" way of how other people typically see things. 10 different photographers can all "say" very different things with their pictures, and all have totally different subject matter, but I bet you that 9 of them will still "say" those different things "within" a very similar depiction of reality: one shooter can do sentimental WPA project photos, the other horrific war photography, but both within a common "reality" of the "traditional" black and white print. Giacomelli's prints, on the other hand, are like from an alien planet, a universe to its own, far more abstract, and far more akin to painting.
You start "painting" with a camera when you diverge from merely "recording" reality, and instead start distorting it or creating your own version of it. The zone system makes no sense to someone like Giacomelli, because he's not trying to be "true" to any outside source or "objective" version of reality.
In this sense, I think "painterly" photographers have more in common with expressionist painters, like Van Gogh or Munch in their intent of what they are trying to do, they dont "paint" what things look like, as much as they "feel" like.