First of all this is not your common kind of whale, Is a beaked whale. As a sort of self-declared (minor) expert in the diet of Ziphius cavirostris I can tell you than this whale is not attracted by fishing nets normally IMHO. This species feeds at very deep waters and do not eat fishes normally. This plastic is sunken plastic.

I had personally get some kg of plastic from stomachs of the same species named in the article. I remember a sac (from manure I think), and a lot of the same plastic bags that you can find in the supermarket. I remember also a small commotion in the faculty by, uh, a lot of diluted blood that ended in the wrong pipeline slowly leaking just in front of the main entrance but this is a different history... don't make me talk about it :-)