The implied irony was that a usability expert produce a site with poor usability, and it's just plain wrong. Visit the link. It's not even that ugly, frankly. It is spare, clean, and very well-organized. It just happens to look a lot like default HTML rendering. Shrug.
I guess it depends on your definition of usability. Jakob Neilsen himself believes that user satisfaction is a part of usability:
http://www.useit.com/alertbox/20030825.html
I agree that it's possible to have eminently usable software that is ugly to a bunch of people. But it entirely depends on how that software is being used.
In the case of a web site, Jakob says it well (from the same article I linked above):
On the Web, usability is a necessary condition for survival. If a website is difficult to use, people leave. If the homepage fails to clearly state what a company offers and what users can do on the site, people leave. If users get lost on a website, they leave. If a website's information is hard to read or doesn't answer users' key questions, they leave.
I would add: if the site is ugly, people leave.
Your web site might be "eminently usable", but if people don't read it, it's a failure.
Don Norman talks about this in his book Emotional Design. Everything we use elicits some kind of emotional response in us, and that affects our experience with the thing. Something is not "usable" if it's so repulsive that no one will use it. In this case, the goal of Jakob's site, for him and for us, is for us to read the articles. If we don't want to look at the page for more than 30 seconds, it's difficult to accomplish that goal.