Maybe I'm just not enough of a fossil, but this approach to software development isn't really anything new. When the author compares XP to "traditional" development, it sounds like he is using "traditional" to mean "the way it was done in the 60s and 70s".
Writing one feature at a time, writing a test for each function, doing code reviews, releasing early and often were all the way I learned to make software back in the late-80's. So, I guess it doesn't seem "extreme" to me. This is just the way things are done on good commercial software teams for at least the past 10 or 15 years.