What do you think is meant by “Abstract photography can be defined as capturing images in which the subject isn’t the most interesting element”?

I think that it means that abstract photography does not need to follow the rules of photography. The photographer can bend and break rules to create images that are considered abstract. This leads to images being created that look wrong but have a unique feel to them. For example, the photograph by Uta Barth uses blur to confuse the viewer so that they don’t know where to look. there is no clear subject which makes the photograph interesting.