Yes it is plural, but it is an English issue to say "you all" or just "you".
"You" is more natural but does not convey the info about the plural. "You all" is clearer about the plurality but is not better translation than "you" alone.
Moreover, there could be cases where a plural you do not involve all, but a plural subgroup. Like even you say ""both of you" or a number greater than one. For example, someone could have said to the Titanic crew and passengers "in the event of a shipwreck there will only be emergency boats for half of you". That's plural but not an all inclusive plural.
Be as it may, "you all" is used in English to mark the plural second person, specially when translating from a language with a plural second person pronoun or verb. It is a convention, but it's not mandatory.
IMO, this exercise's translation and many others don't need the "all". You may add it, it would be fine and useful to better identify the plural form, but probably never the best or most preferred translation. Of course if you report it is OK and should be eventually included, but it is for educational and linguistic purposes and not the most natural or common way of saying it.