Unfortunately, not many others would be willing to put much faith in someone with the grammar of a 6th grader to do a better job at UX design than a multi-billion dollar company.
There are further reasons it's not an ad hominem - I recommend you read The Ad Hominem Fallacy Fallacy [1]. I don't mean to criticize - I made the same mistake until recently, so I offer the article in a spirit of goodwill, not superiority.
A: "All rodents are mammals, but a weasel isn't a rodent, so it can't be a mammal." B: "Well, you've never had a good grasp of logic, so this can't be true."
Maybe I'm not seeing the difference though (not a native speaker, just curious).
The example you show instead points to an attribute of the speaker to imply that what he says is categorically untrue, regardless of whether it's relevant or not. In this case, an attribute of the speaker is used to refute a specific argument of his (that he is such a demigod of UX design that he can design better than any of the top teams in the industry), while still allowing him to participate in the debate.
No argument can be made connecting someone's poor grasp of logic to the taxonomy of weasels, but an argument can be made connecting someone's poor grasp of grammar to that same person's ability to design.
(I agree with your gist though about ridiculing the idea it would be trivial to do a better job at design)
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.