“Colon-separated files” are not unstructured though. And arguing that s-expressions are more expressive than text because “text” excludes all structured examples of text seems like an excessive in definitional acrobacy acrobacy.
The goal of the definition isn't to give S-expressions an advantage so they can be proven superior and Lisp people can win the argument. It's to describe the sorts of files that actual existing command-line utilities (like this in Unix/Linux) can conveniently operate on.
The "head" command deals in line-oriented fashion and can print the first N lines of a file. The "cut" command can select certain columns based on a line-oriented and character-delimited structure. There is no exact definition, but this gives you a general idea of what text files means in the context of "becoming a command line wizard".