I'm confused by this comment. In this article Gruber is saying how much better iPhones are than everything else. He doesn't seem to take the biggest criticism of "no fixed keyboard" seriously.
He has taken the "no fixed keyboard" argument seriously. He wrote about it a number of times. He came to the conclusion that (when implemented well) there is not much of a difference, except user preference.
It just depends on your definition of "taking it seriously". For example in that article Gruber says
Most people can thumb-type just as well, if not better, on an iPhone as they could on a BlackBerry.
I prefer the iPhone to a Blackberry overall, but the argument that there is no advantage to typing on a Blackberry is just not true. Here's a better analysis:
That's not an "analysis". That's an opinion just like Gruber's - who, incidentally, has much more in-depth articles regarding thumb-typing than this one, which treats it as an incidental issue.
http://daringfireball.net/2009/07/mobile_phone_keyboards