I don't think it's overstating it at all. If anything, I would argue that code is more expressive than literature or music because it's more general. Consider the space of all possible video games, demos/intros, generative paintings, generative music, etc. which wouldn't be possible without code.
I think you're confusing the common use of a medium with expressiveness. Many uses of ordinary language also aren't that interesting, but we need to consider the full space of possibilities.
I think you're confusing the common use of a medium with expressiveness. Many uses of ordinary language also aren't that interesting, but we need to consider the full space of possibilities.