Jeremy, please teach me, why it should be Requires(pre): /bin/sed, when sed is
actually used in %post? At the moment, there's Requires(post): /bin/sed, this is
wrong?

Whoops, yeah, I just typo'd.

The problem is actually that you've introduced a dependency loop.  Both sed and
info require each other to be installed first.  Given that the sed invocation
was added to fix something cosmetic, it should probably be removed or at least
just be
  [ -x /bin/sed ] && /bin/sed ...
without the requires

OK, thanks for clarification.