[sws from rfc]

  By following this algorithm, the right window edge may not be advanced
  by duplicate ACKs in your example and in the trace of the original post.

The conditions under which the window is not advanced are different.
In the spec you quote, it's about not making small updates to the
window when it is substantially closed.  The case under discussion is
about only making updates on packets which advance the ack pointer
(this should not happen if window was fairly closed and is being
opened a lot, though).

-- 
        Greg Troxel <gdt@ir.bbn.com>