The irrefutable fact is that the author presents the core logic of the file-reading solution as a defn, not a defmacro. It may be driven by a macro, but evidently that part is not noteworthy enough to be presented.

I mean, sorta, but Lisp macro writers learn early on that macros are also functions, it's just they operate on a list of symbols, and are called in a different phase.

So saying "it can't be a function" doesn't ring true, either.