Although the article is riffing on "nosh rubs itself on a food blender", there were not really very many samples of that behaviour seen. 106 rubbings in total,

Although the article is riffing on "nosh rubs itself on a food blender", there were not really very many samples of that behaviour seen. 106 rubbings in total, some against a shark and some not - unclear, let's say 50. They mention over 100,000 individuals seen. I'd venture that evolution isn't involved at all and it is simply opportunism that was observed. It's such a rare event that we can't science this one much. Before you grow fingernails, you need to grow fingers. Before you grow fingers, you need something to wave them around on otherwise they don't do much - that'll be arms (or legs). Fish have been there, seen it, done it (multiple times) and reverted too (multiple times).