If you read enough on Ken Rockwell's site you will come across a few references he makes himself to his deliberately "provocative" statements - made specifically to stir up controversy and stimulate comment (and, not incidentally, to bring more visits to his site by folks wanting to see if he really said what others are quoting -- advertising is sold based on the number of visits to a site). Although there are many who do not consider Ken Rockwell worth reading (and I am one), I know enough about him to know that he is aware that lens distortion in virtually all lenses is greatest near the edges, not the center. So it is pretty obvious that this particular statement was made deliberately in the hope of generating discussion (and more site "hits").
By the way, I found Ken Rockwell's site many years ago and read a lot of it. Then I happened to read a comment left by an individual about a lens review that Ken had posted. The comment was questioning Ken on some specific factual errors (rather glaring errors) in what Ken wrote in his physical description of the lens. In response, Ken admitted that he had never actually had or used the lens and wrote his review based on advertising copy and other reviews he had found online. I stopped reading his stuff after that.
When asked about K. R. I always suggest that anything he says should be taken with a large grain of salt, and when he says something that goes against all previous testing and facts it needs to be recognized for what it is - a deliberate attempt to stir things up and to bring more traffic to his site.