Text is a visual interface, if you look really closely you can see each letter is actually a little picture.
Umm
Can you use text to move a slider?
Yes.
(FWIW I have watched and enjoyed all Bret Victor's videos, and am looking forward to his big reveal. I've also experimented with visual front-ends to tidalcycles, so agree with you to some extent.. But text is really good for describing pattern.)