Is the subject not the only one who doesn't like the object or is the object not the only thing the subject dislikes?
Which of these are good context for this sentence?
1) Mereka tidak suka dia. Kamu juga tidak suka dia.
2) A: Dia tidak suka saya. B: Saya juga tidak suka dia.
3) Kamu tidak suka Andi. Kamu juga tidak suka Sri.
'Also' in English works on all of them.
I don't think "them" is universally understood to be an epicene (non-gender-specific) pronoun replacing 'him' or 'her'. So you probably shouldn't try to use that in DuoLingo.
But you're right. It is a valid way, in English, of referring to a person without mentioning their gender. I would say you should avoid it in more formal contexts, though.