Worse, porn was assigned to be a causative factor rather than an associated factor. These girls were already deep in trouble. Porn does provide 'social permission' for all sorts of evils and activates hormonal responses likely to increase the drive for sexual activity, but that's hardly news, nor does it explicitly indicate that it motivates group sex.
Sounds more like a these researchers were a bunch of closet pervs looking for something 'fun' to study.
If you read the whole article, various studies were cited with various conclusions. It wasn’t just one study.