That's a pretty trite response. I can think of a few reasons:
1) Maintainability. JS is probably easier to grok in some circumstances, ie. you don't have to send new devs to a blog post to explain the CSS on your project.
2) Cost/benefit analysis of dev time, ie. you could do X (where X is some minor display that 1% of your users see) in pure CSS in 12 hours, or do the same in 30 min of JS.
3) Cost/benefit analysis of page size, ie. you can do in 4 lines of JS what would take 400 lines of CSS. Sometimes a few lines of jQuery obviates several hundred lines of CSS (between the media queries, browser-specific overrides, and so forth).
The interesting question the article should address was why the CSS solution was a better choice than the JS solution. I have a gut feeling the answer is way better than "we just wanted CSS only for purity's sake", and personally I'm interested in knowing what that answer is!
Well the thing is, none of your reasons are applicable here. He has a valid question as to why you would need JS as this is purely a styling between mobile and desktop. The only thing you could potentially do with JS would be to strip the content and rerender it which would be very strongly tied to the HTML structure. Once again, your points are valid, mostly, but for this particular problem, you wouldn't need or even want to do it via JS. I think that's the solution to the original question.
I don't think there's a reason why he chose to not go with JS, but to show that it's possible with only CSS.
IMO I like using less JS because it slows down the Mobile web experience. JS is super powerful and you get to do cool things with it but you always have to keep an eye out for performance.
