This seems to slip by slightly in the article.
XP has (at least) one organized body of information
on continual improvement called "Refactoring."
You can accept that there is one and only one
Or you can chose to disagree with aspects of this
body of knowledge, seek to improve it, or maybe
substitute a new body of knowledge, but still
continual improvement needs to be emphasized as
it is part of the trade-off involved in not doing
the big design.
But in either case, don't both programmers need
to at least accept the goals of refactoring to be
able to work together?