What makes Mr. Hockney's claims absurd is that many classical realist paintings are, in many ways, superior to photographs. A photograph depicts a scene as one would see it if one looked through a peephole. By contrast, a painting can represent a scene more as one would actually look at it, moving one's head around and shifting one's focus from one part to another.
While it's possible that some artists might have benefited from being able to experiment with a camera obscura, to get a feel for how certain three-dimensional shapes might translate to a two-dimensional painting, they would not have used a camera obscura directly to produce their works. Bouguereau occasionally painted people from photographs when a live subject was unavailable (e.g. if he was hired by a decedent's next of kin), but he much preferred live models. The fact that Hockney could use a camera obscura to produce some bad paintings in no way implies that good painters used such devices.
That was exactly his evidence: a lens sees things differently from the human eye. The paintings in question have the effects and perspective of a lens.
***A photograph depicts a scene as one would see it if one looked through a peephole.***
The cameras also distort the scene if you don’t use enough distance to photograph them.
Often people who paint from photos will use a distorted photo in which a telephoto lens has been used bringing the back mountains way too close to the front.