From the article -

it's impossible to draw definitive conclusions about what's causing the skew toward topless pictures.

One explanation could be simply that other users are engaging with these pictures more, prompting the algorithm to place them higher in people's feeds. If you and others "like" a photo more, it could appear higher in other people's feeds."

I honestly can't bring myself to feel even slightly surprised by this - machine learning/algos if successful should amplify user preferences. I have a strong feeling this is fueled by underlying user preferences rather than some ill-intent to push semi-nude pictures.