"You all didn't drink tea"
When they put "you all", they're really just asking for the plural you. A lot of people use "you all" that way, so they put it in as a valid translation. They've obviously also used it in a few English to Swahili sentences, which creates a problem when people then interpret it as "all of you", which isn't quite the same as "you all".
I think your translation fits in the "all of you" category, I'm afraid.
I speak Swahili and I am just taking a sort of refresher course to improve my vocabulary and correct my grammar. The way I wrote it is the exact way you would also say "You all didn't drink tea" as well as "Hamkunywa chai" the only difference is that maybe mine is more stressed on the "You". Besides I don't really understand your differenciation of "You all" and "All of you". Their meaning is quite the same as I interpret it.
I'm familiar with "you all" as a US dialectal way to indicate plural you and I correctly guessed that it was used here to indicate just the plural and not stress the "all-ness", but as someone who doesn't have "y'all", I distinguish no difference between "you all" and "all of you". If I say "you all" or "all of you", I mean nyinyi nyote. (If I just mean nyinyi, I would never use "all" there ... I would say "you guys" or "you lot" to make it clear).
Personally, I would never say this sentence this way though. I would say "None of you ..." "All of you/You all didn't do X" sounds really awkward to me and I'd always say "None of you did X," but that's an aside ...
So, this is basically a long-winded way of saying I agree with you. Anytime "you all" appears in a sentence, nyinyi nyote should also be accepted because it's not simply a plural marker to all English speakers.
Also, throughout this course ninyi tends to be used and I don't think nyinyi is accepted everwhere it should be, so if you feel like doing the course a favour, keep typing nyinyi and reporting it whenever it only accepts ninyi.
I am familiar with 'y'all' but your explanation has enabled me to distinguish the difference between it and "all of you" Thanks for that. However, I never thought of it in this instance. I'll make sure to report the "nyinyi" everywhere I can as I have never heard it used in my dialect of Swahili.