It might've been something I just extrapolated then. Either way I think it's a reasonable assumption that they'll give you some kind of note editor. For instance, APB had a simple in game note-editor (along with its clothing editor) that would let you play a little jingle when you killed someone. You could also buy and sell songs on the market. They did mention there would be opportunities to perform in public and with others which means they should flesh it out more than they would with some more trivial features. With a mouse you can't do chords or really play properly and people aren't going to be very interested in playing. Practice all you want, not gonna sound good. At the very minimum they'd let us map keys to notes so we could use a MIDI keyboard plugged into our computer (or just the computer keyboard of course).

I was under the impression they were going to let us upload or record MIDI files (or something similar) to do the actual playing. I think they are aware no one can play an actual song with a mouse.