Why is it not "Ihr seid alle langsam"? Where is the "all" gone?
It is not necessary, ihr always addresses a plurality of people.
(DL seems to use you all in order to specify a plural you.)
"all" specifies "everyone", and this is not included the plural "you". "you" does not necessary address everyone in a group. Anyway, even then the "alternative" translation should be accepted as well.
This sentence is from the Pearson cooperation: https://www.duolingo.com/comment/24066422
I believe they like to keep du and ihr apart by translating du with "you" and ihr with "you all".
Please direct further questions on this issue to them.