It's not a "synthesizer" that makes sound but one for visuals. You'll make your own sounds somewhere else/somehow, pipe them into Hydra and have it react to your own music/sounds.
I'm guessing they don't call it just that because of the modular nature of a programming language and because it depends on the user configuring it, rather than just hitting "Play".
I'm not super familiar with Advanced Visualization Studio but as far as I can tell, it wasn't geared toward live programming in a performance setting nor was able to work with general libraries as it was it's own language, not using a general one.
I think they're a bit different, a visualizers uses incoming audio to create visuals, a video-synth can do that but it can also do much more, without the involvement of sound in any way.
It's not a "synthesizer" that makes sound but one for visuals. You'll make your own sounds somewhere else/somehow, pipe them into Hydra and have it react to your own music/sounds.