The "code is data" slogan also misses the point. Text is data, so all code is data, whether or not it's Lisp code. (The only code that isn't data is code that has been compiled down to hardware. In the olden days computers were programmed by plugging wires into plugboards. That code wasn't data. But any code that is rendered in the same medium as the data it processes is data, and nowadays that includes all code.)
What matters is that text is structured fundamentally differently from trees of cons cells. Text is a vector of characters. It is a fundamentally flat data structure. It is well suited for humans to read and write with pencil and paper, or chisels and stone tablets. It is not natively suited for describing hierarchies of abstractions, which, it turns out, is what you want when you're writing code.
What matters is that text is structured fundamentally differently from trees of cons cells. Text is a vector of characters. It is a fundamentally flat data structure. It is well suited for humans to read and write with pencil and paper, or chisels and stone tablets. It is not natively suited for describing hierarchies of abstractions, which, it turns out, is what you want when you're writing code.