From the article, it seems like the largest problem was dumping of wastewater overflow that was beyond processing plants’ capacities. As part of this push to clean up the Seine (as part of a PR for the upcoming Olympics), they’ve built a large reservoir to temporarily store waste water overflow until things level off and the treatment plants can treat the water normally.
It sounds like a great step forwards in infrastructural improvements that will make a huge difference to the ongoing cleanliness of the water. I share many of your feelings, having seen the Seine myself a few years ago, but this article genuinely makes me feel hopeful