In this context, think of "text" as shorthand for unstructured text. Which is another subset of text. The sort of text that Unix tools traditionally handle. Colon-separated files like /etc/passwd, line-oriented files like /usr/share/dict/words, etc.

So, now we have two subsets of text, and it makes sense they can have different levels of expressiveness.

(Actually, unstructured is too extreme since there is a little structure. Something like "isomorphic to CSV" isn't quite right, but closer.)