so what happens if you display an extremely high res image of a 100% AI generated fake-something on an 8K display in a photo studio room and take a picture of it with the camera? it gets tagged as authentic.

Was photo not authentic? Think of it "as seen by an iPhone", not "this is authentic event" verification.

But Apple likely would reject such photo because of inappropriate depth map / LiDAR data.

The timestamp wouldn’t match the event being depicted and the geotag would show the studio. And the depth sensing would show the image as flat.

That’s a lot of money and effort for a fake photo