Who wants to learn Chinese(Cantonese) here?
To the best of my knowledge, this entire line of thinking is based on a comment Luis made about Spanish, that they wouldn't have courses for more than one Spanish dialect. At least I have never seen any direct comment at all about any other language. Yes, logically what applies to Spanish applies to Portuguese, English, Dutch vs. Flemish, Catalan vs. Valencian, but it's a leap to assume it applies to Chinese, Arabic, Italian "dialects," all of which are linguistically aren't dialects at all. Neopolitan and Venetian duly appear on the request list without comment, for example.
Regardless of the thinking behind it, I am pretty confident that the exact text was OK'd by staff, before it was added to the guide. I really doubt that moderators would make up policy by themselves - this will have come from Duolingo.
Why have they chosen to single out these languages? I assume because the naming can cause confusion. So they are clearing up that when they are discussing adding Arabic and/or Chinese that they don't mean all these other dialects/languages. Example: If I said to someone that I was learning Arabic, they might ask me: which one? Same situation if I had said Chinese. You don't really have this problem with Spanish/Italian as the dialects have a more distinct identity (to English speakers). Note: This is my own assumption - I don't really know why.
I'm hoping someone can comment on this question I've been wondering about: what writing system would a Cantonese course use? I'm under the impression that Cantonese speaking areas largely use Mandarin for writing and that written Cantonese isn't actually standardized as yet, as it's mostly a spoken language.