I'm not saying that it is an ad hominem, but I've read through the page and I think the connection between grammar and design skills is a red herring. The page considers this an ad hominem too, even though having a grasp of logic is clearly important for making A's statement:
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).
As I've mentioned in another comment I just posted (after your comment, that is), it's not an ad hominem regardless of whether the "poor grammar -> poor design" argument holds, because there is an argument present, not a simple dismissal of everything he has to say. It might be wrong, but it's still a valid argument, and not an ad hominem.
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.
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).