It's just a convenient short hand for purely AI generated and (hopefully) human edited content, in contrast to blended or AI assisted content. The slop is a semi-ironic way to show you don't have a stick up your ass or chip on your shoulder about it, and also an acknowledgement that there is a lot of garbage out there currently so the default presumption of quality level is just "no". Obviously I think it has value since I shared it, and I'm not here sharing any of the modern traditionally produced mainstream garbage singing its praise.

I don't think I'm alone in using it that way either, given that they named one of their own other projects Cream of the Slop.