So, to the point. Aza argues that you should use undo instead of warning because undo is, despite the prevailing opinions on the subject, easy to do in web apps. This is just plain wrong - a general solution to undo in AJAX web apps is a hard problem. He's off the tracks quickly:
In this series of blog posts, my goal is to explain just how easy it is to provide Undo functionality
So, you need a series of posts to show how easy something is. Ok, if it's so easy, why does it take a series of posts?
Next, Aza deploys a commonly used (and completely incorrect) approach to prove his point: he offers up one specific example where his conjecture holds. This is about as clever as saying "Everyone drives a Honda. See, I drive a Honda". To prove that undo is easy, you can't just show that in one specific example it is easy, you have to show that it is easy in all situations. The comments are ample proof that there are scenarios where a solution is tricky.
Aza continues, either out of ignorance or religious devotion, to defend his position by pointing out that his example is only valid in a specific, simple scenario. This amounts to defending your position by agreeing you are wrong.