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".

Is this much different than Winamp's Advanced Visualization Studio? That was also 'programmable', depending on how you define programmable I suppose

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.