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.
Nowadays, servers that are not connected to a monitor, keyboard or mouse are often referred to as headless. Regardless of whether they have a graphical user interface (which can be used with tools such as Guacamole, for example).
I'm not trying to say that this is correct, but simply to point out that the term "headless" is now often interpreted differently.