For others who wonder the same: the announcement is from the 19th this month and they licensed it under Apache 2.0.
This is really cool to see! I hope they are testing the waters for releasing the source engine as free software. I remember Gabe Newell stating that they would consider it if people were interested. Doesn't have to mean anything, but a woman can dream~
It's more than just a simple tool with which you can tweak audio. Some features are listed at https://valvesoftware.github.io/steam-audio/. The tool is mainly intended for the development of games (also under Linux).
And yes, it's basically just about the fact that Steam Audio is now completely open source.
It's a tool for easily creating realistic audio in games. Basically you can give it a 3d environment, and it can bake sound propagation information so that sounds have realistic reverb and filters. They've used it for some of their recent games (most notably Half-Life: Alyx).
God the sound design in Alyx was insanely good. I felt like I was legitimately in City 17 and it was terrifying. It was a really good showcase of what Steam Audio can do.