“Abstraction photography can be defined as capturing images in which the subject isn’t the most interesting subject.”
I think this quotation means that abstraction in photography is not highlighted or created as an image by what the audience can plainly see. Instead, all the elements within the photos, every aspect and pixel, work together to create a harmonious image. I think the quotation implies that abstraction goes beyond what you can see to question the image, the shadows, the colour and the chosen aspects to understand the intent in choosing those specific objects and the purpose of an image.