Although it's easy to sympathise with this criticism, since there's no doubt that #DocBook is a beast and can drive you mad, this rant is also caused by the author's perspective.
If you use #DocBook as a source for producing man pages, it reminds me to learning #org-mode for writing your shopping list. Man pages have a very rigid structure, but the main strength of docbook is its flexibility. And with this flexibility comes ambiguity and redundancy.
If you look at the discussions between the people involved, it's not that they didn't see the problems, but it's about how #DocBook evolved. It's true that it was originally designed for writing "technical documentation". But what is technical documentation? My main use of it is for writing projects that are not "technical" in nature - but where the structure is often unpredictable. And as much as I've wanted to get rid of DocBook, I keep coming back to it because it allows me to even misuse it for purposes it wasn't designed for.
I can see a parallel there for why I like #Perl (which most people seem to hate as much as XML in general). I'm not a programmer, and I don't care if you have trouble understanding my code. But it gives me the ability to do things my way without having to rebuild everything from scratch. For me, these tools have certain values that I miss in most of the others out there.
And to make things even more complicated: I love #OpenBSD and hate man pages ;-)