This is all made possible by a rather lovely piece of software called SuperCollider, which is a programming language specialised for sound and music. The really nice thing about that is that it has the benefits of "traditional" music software on the one hand (for example, standard effects like echo and reverb are easily at hand), and also the benefits of a modern programming language on the other: it's much more flexible than traditional music software, you can tell the computer abstract things like "play these 7 notes, then a random burst of noise, then 12 random notes of your own choosing, then repeat the whole thing 4 or 5 times". (That description doesn't sound like it'd make a great piece of music does it? :) Well let's just call it an illustrative example.)
Programming languages can be a bit scary to learn at first, and I think it's perfectly reasonable that more traditional music software is still used for a lot of things. Often, music producers have a standard way of working and a standard set of effects that they pick off the shelf, twiddle some knobs, and there you go, no problem.
But as a fairly experimental musician I long ago got fed up of those restrictions and searched for something that would give me a lot more freedom. The flip-side of that freedom is that sometimes you have to spend more time building the tools you want before you use them. But I've been able to do a lot with SuperCollider, to the extent that it's now my main tool in my PhD work where I'm processing beatboxing sounds in real-time and transforming them for interactive music purposes.
The SuperCollider tweets thing is interesting and fun - 140 characters is a very strong limitation so to try and create even a decent sound, let alone a whole piece of music, is quite an achievement. On the other hand, SuperCollider tweeters often resort to clever little programming tricks in order to squeeze as much as possible into that tiny space.
So I do worry that the sc140 project makes SuperCollider look almost completely impenetrable, these 140 characters of obscure alien code! To demonstrate that SuperCollider's not actually too scary to learn, watch this SuperCollider in 60 seconds video.