This leaves out part of the clause.

  All identifiers that begin with an underscore are reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

You're right, and I guess I've been breaking that rule for a while. What's the purpose there? The double-underbar and underbar-capital rules seem to be allowing for non-conflicting introduction of keywords. Is the single-underbar rule to protect standard library headers or something?