I may be assuming unfairly, but the than/then confusion referred to is a problem I have seen almost exclusively in native English speakers who try to spell phonetically. Non-native speakers have other problems, which I generally gloss over.
It seems a fair assumption that mattermill is a native English speaker when he's on a primarily English-speaking site, committing a primarily English-native grammar error, and echoing some very American hubris (not saying Americans are all terrible - I am one - but we do have a certain characteristic braggadocio).
You can make something pretty without the ability to clearly communicate and a strong attention to detail, but you can't effectively design for information and interaction purposes without those skills. Anecdotally, most of the excellent hackers and designers I've met have had excellent communication skills, and those native to English usually speak it impeccably.
Also, the kind of absolute mastery of UX design he claims (the ability to out-design all the best in the industry on his own, based only on screenshots) would require all but perfection of all the attributes of a good designer. Such a trivial mistake would not pass the muster of such a demigod of design.
Regardless of whether I'm right in that argument, it's still not an ad hominem, because there is an argument present, not a simple dismissal of everything he has to say because he made a grammar error. It's a challenge extrapolated from his grammar to his implicitly claimed mastery of UX design, not a dismissal of his ability to participate in the debate.