What is important may not be specifically that Wall pressed the shutter. After all, we don't complain when a director is not behind the camera of his movie. What is really important is that Wall is the one making the work in the medium of photography, and by that I mean taking the important decision that relate to the picture: subject, light, composition, etc. Those are different than performing the actual practical tasks.
But of course, there is a point at which artists who make mise-en-scène must acknowledge their staff in the making of the work. I think it is rather an abuse of language to call a photo like "Invisible Man" to be only a Jeff Wall picture, just like it would be a stretch to call a Steven Spielberg movie the sheer product of his effort. We all know that important painting masterpieces were produced in the studio way, with assistants, and apprentices doing the menials.
Come to think of it, when you are making a portrait with a model, you're already doing collaborative work.