It's hard to completely tell from the article, but this almost sounds like they built a new product, not really a rewrite. The new product subsumed the old system, but really could be described as a different product (in the way that Word is not just a refactored version of Notepad). Nevertheless, great read.

One thing to also note is that Fred Brooks has stated that his "Throw one away" idea was wrong. He now says, "I would now recommend this for bad teams only. Now, I suggest you make schedules on previous experience of your team. My new approach: INCREMENTAL SE, i.e. build a small functionally limited but working system and then expand it."