They are not that secretive, it's been written about several times. Long story short: a lot of it is still Glenn McDonald, the person behind everynoise, that was acquihired when they bought echonest.
Here are a few articles about it (there are several more): https://artists.spotify.com/blog/how-spotify-discovers-the-g... https://www.thefader.com/2020/12/02/escape-room-genre-spotif... https://www.thestar.com/entertainment/2016/01/14/meet-the-ma...