"Write programs that produce and generate text, because that is a universal interface" is not a good principle for a compiler. (In fact, I think it's not really a good principle all around, and Unix is worse for it, but especially for a compiler.)
The author of the article is not writing a compiler.
I fail to see how that quote has any contextual connection with the current article or compiler design. A discussion about the merits of Unix text streams or text in general is another discussion entirely then the current one.
You yourself said "there is really no rigid boundary" between a compiler and a preprocessor. By most practical definitions of a compiler, the "C amplifier" discussed in the OP is a compiler, and is definitely similar enough to suffer many of the same issues as a compiler. In particular, I think what pcwalton is getting at is that in such a "pre-processor", it's likely that you'll eventually want more expressive data structures than text can comfortable offer you.
